I have an 86 camaro with a 305 in it. Rebuilt top end, still no performance.
I found a 327 block with heads and everything else on it except a top end. The guy I'm going to buy it from says to bore it .30 over and put a mild cam in it and it will blow that 305 away. Is this true?  

There is much more potential in a 327 than a 305. Think of it from a specific output scenario. If your rebuilt 305 is making about 250 horsepower then that's about .82 horsepower per cubic inch. Not only do you gain 25 cubic inches you increase the potential output because the 330 can "breathe" better through a larger valve because of it's larger bore. So let's say we gain 10-15% in specific output because of the benefits of the 327. So we can say in relatively the same "condition" as a 305 a 327 can make a minimum of .90 Hp/C.I., and wallah there's another 50 horsepower.  

So then it's probably a good investment? I am a daily driver though so will it still be okay on gas mileage? For some reason, I'm only getting 12-13 with this 305 which makes me think something is wrong with it mechanically.  

As Gib stated,a 327 is stepping up.The only loss you may see is low end torq due 2 less stroke,but the high end gains will tip the overall in your favor.As to the 305 mileage,alot to b considered.You gave no detail of the mods,if any.But a properly tuned 305 should do much better than 12-13 mpg.Sounds as if something is way off with the tuning.  

The 305 has a 550 holley carb (needs new jets and power valve is probably blown), edelbrock performer manifold, edelbrock air filter (nothing special), and 3' flowmaster cat back system. That's all we've really upgraded. That and distributor, fuel pump, air conditioning, alternator, etc.  

I f-n hate holley carbs cause of them power valves.Havent used 1 in over ten yrs.That power valve being blown will cause all kinds of shyt.You seem to know that already.Youve got the right size carb 4 it at least,but holleys are jetted rich on assembly,so you prob need to lean the jetting for economy.whats ur dwell set at? Id say tweek the heck out of it on the tuning side,but if ur puttin a 327 in soon dont go pocket crazy trying to get it economical.If you can,Try for some dual exhaust.Youll need it with the 327 anyway to get some low end.  

The 327 is definately a step up.  Funny thing, the wikipedia lists the most powerful 305 at 250Hp and the weakest 327 at 250Hp, but those numbers really don't mean anything due to the fact each engine was built for different reasons and in different times.  My friend has a 327 4V in his '68 Camaro and it doesn't have a problem burning rubber into 3rd (4spd) and nothing has been done to the engine except ring and bearing job in the early 80's.  

The damdist thing aintit.Their no low end torqers,but windem up and theyll sure get movin.The only thing I never liked was just that,you gotta gear em STEEP and wind the heck out of em,but they  will do the task.Thats the dif I see most is just like any engine having the bore and stroke so close in size,the 305 pull,s so nice cause of torq. at the high end and with less piston weight it can sustain high rpm for greater lengths of time.it supprizes me nobody has worked with it more to make a set of heads to flow better.Buick had kick ass results with that in their 350's.  

Well I already have a 3" flowmaster catback system on it now.  And I looked into power valves and they say the new ones last for 1 million miles   .
But I'm thinking of getting the 327, if it really is going to be a good step up from a 305..  

A catback is just that.You still have the heavy backpressure loads @ the cat. conv.You never open up the exhaust were it needs it,at the HEADS.to make low end mid range come out,you gotta get the sqeese out of the cylinder fast.The more sesistance the piston has on the exhaust up stroke the more it slows down,hence the loss of force of motion @ the flywheel,torq. 327's produce great hp rates,but fall short on torq due a shorter stroke.So do what your able to to gain more befor you start taking it away with a cam.A 305 is an engine made to produce lots of torq on a small scale. 327's the 0pposite,made to rev and make hp on top end.Two different breeds altogether.If you dont have gains there on the forehand,youll miss those loss's bigtime.  

So what do you suggest I do?
I took it to a shop to but true duals on it and they said their is no way to do it on my year model (86) because of the floorboard. We had put headers on it but took them off because pops said it would be too hard to change spark plugs, and work on the engine.  So what else can I do to make it breathe better and run better? I'm assuming it will run better than the 305 anyway.  

That shop I would never go back to first off.Ive put duals under most the cars Ive had,cutlass,monte's,chevelles.Its able to b done easily.My dad owns a tubing bender,and its time consuming but not hard to do.That is your best option to bring back the loss of torq from less stroke. 2 1/4 or 2 1/2 tubes off of a good set of headers.Exhaust is were your torq is gained back,starting at the exhaust valve.Optimize your backpressure and you have optimized your torq.There IS a science with it.I tel all open the thing up as much as you can from the valves out and build backpressure at the mufflers.smooth street perf comes mostly frome smooth torq. An object in motion tends to stay in motion,so the more motion you keep from torq the more gets to the ground.It all builds off of smooth flow at the right rates 4 the engine.  

Well i'm only 16 and i don't want to invest thousands into a custom dual exhaust kit. Eventually i want to do true duals but my dad hated working on the engine when it had headers because he said there was no room. If i could find somewhere that i can get everything set up and duals put on for a reasonable price i would..

Oh, by the way, they said they couldn't do it because their was no where to put the mufflers. Something about the traction bar being in the way.  

I understand being young and needing help,and its way cool to have dad helping on all spectrums.If you have the intent to keep the car,keep the headers around till you get good enough knowhow that dad doesnt have to do that little stuff 4 you.Then putem on again if you like.manifolds arent badly restrictive on a stocker,so if you can get hold of a tube bender,true duals are not bad for material costs.My entire system was under 200 bucks doing it myself.But I have some good equipment at my dispencement. As to the shop,I get the impression they were blowing it off to avoid the headache  

Well i just called a shop and they said they can do it for $550, from the headers back, and that's including pipes. Now i have to convince my dad to let me get the 327, set it up, put it in, slap headers on it, then take it to the shop.. hmmmm.