Tranny and Diff match up. Help! This is my first build .

Wow!
This building a ride is more complicated than I expected.  I need the help of the experts.   Here's my story:   I've been working on a grad present for my son for the past couple of years and need to get it done ASAP. It's a 1935 Olds pickup truck built from the remains of my grandfather’s 1935 Oldsmobile 4 door sedan that was chopped into a grain hauler during the 1950's. I've had a Nova front clip merged to a 1947 Ford PU frame, added a 1980 Camaro diff (2.56:1 ratio) rebuilt but stock 3054BBl from a Buick Regal with what I thought was a TH350 lockup but it turned out to be a TH250. As the TH250 is prone to failure I want to upgrade the tranny to a TH350 or 700R4 before I have the truck painted. The 700R4 appeals to me for its better gas mileage and its well within my capabilities to adjust the trans mount etc.  However the more I research the various trans to diff ratio combinations the more confused I become about a suitable set up. I must admit I did not realize the Camaro diff was a 2.56 until today.
The truck will run between 25 to 27 inch tall tires (tire height) and will see more hwy than city roads, zero strip usage or attempted burn outs. What would you recommend for a tranny to run with the 305 SBC and 2.56 Diff to obtain acceptable gas mileage and adequate torque? The engine will remain in place for the next few years and I'm really not keen on changing the differential if it can be matched it to a suitable tranny.
I look forward to hearing your recommendations.
Cheers, Bj  

700-4R has a very deep first gear...it'll help quite a bit.  It also has a lower (higher numerically) overdrive gear than a 200-4R...which can help with that very high (lower numerically) rear gear.  A 200-4R will also work...but you may end up being happier with how a 700-R4 drives with that rear gear.

I have an 85 Delta 88 with an anemic 307 that was backed by a 200-4R and 2.56 rear gear.  Thing has 178000 miles on it...and the only reason I've pulled the original 307 is to drop in a big block.  It made a very nice cruiser...slow as heck...but a nice cruiser.

For something that'll never see the strip...this combo should work fine.

Good luck.  
68 Olds Cutlass ~ 350 Rocket
85 Delta 88 ~ 425 Rocket
02 Silverado 4X4 ~ 5.3L

Great advice; thanks for the feed back.  Maybe I should consider leaving the TH250 where it is and change everything once it burns up.  Ultimately we want to run a 350 SBC matted to a 700R4 and rear diff of  3.08:1 to a 3.36:1. But for now we just want to get the truck assembled.  Check out my pic of the project on Readers Rides taken just after the Olds  initial mock up.
Bj