NEW CHEVY 305 CRATE MOTOR PINGING ON 87 OCTANE

I just installed my new Mr. Goodwrench 305 motor. It is a stock '88-92 motor with a factory roller cam and stock 9:1 compression. I have a Rochester 4bbl carb with an HEI distributor. I have the static timing set at 7 degrees advance. The trany is a 700R4. I am using this set-up to achieve fuel economy in my daily driver-'66 1/2 ton short bed. Obviously, I don't want to buy higher octane gas. Going up a slight incline in 4th gear with lockup in trany, at 40mph the engine rattles something terrible. I haven't checked total timing but I believe my "swap meet special" HEI distributor(new,in box)China vintage distributor has stock mechanical/vacuum advance. Any ideas?  

I don't think timing is the problem. Your problem lies in your gear selection. You are in overdrive with a .7:1 gear ratio and are trying to push a 4000+ lbs vehicle up ahill at practically IDLE. Your engine does not make enough power to push that kind of load, you are lugging your engine. try it in third gear or without the lock-up converter engaged. You could try switching to a 4.11 rear axle, if you wish to drive up hills at 40 MPH in 4th gear. If that is the only time you are getting vibration, I'd say it is from lugging the engine. If it was timing, yu wold most likely get a part-throttle ping even on flat ground. That would occur from either over advanced timing, or lean fuel mixture. Of course I cannot understand how you spent all that money on a brand new "vintage" Mexican cast short block, a 700-r4 conversion and you are complaining about $.20 more per gallon for unleaded premium. I run all my cars on unleaded premium. I have very little issues with either of them. In fact since premium has more resistence to detonation, you can advance you timing further, to achieve better low end response. That is good for a heavy truck that needs low-RPM torque to get moving and stay moving.  
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Thanks for the reply. Much of your reply would  be accepted if this was my first build  using these components. My previous fabrication used a '64 Chevy truck with the same rear end ratio-3.78:1-,305 with the same carb and distributor. With 87 octane gas and my "idling up inclines" did I get even the hint of pinging. Also, the truck weighs 3400lbs.  

Ok your title says pinging, your post says rattling. I take rattling as truck rattling from engine lugging. Pinging is something different to me. I can hear pinging. Timing would be your prolem then. Try a vacuum guage hooed up to the full vacuum port and monitor engine vacuum during acceleration. Over advanced timing bviuosly burns the fuel too fast, thus requiring you to hit the gas harder, increasing air flow through the carb, and dropping engine vacuum. Try to find a timing set that gives around 10-14" Hg with light throttle acceleration(maybe 10% throttle if that). Check cruising vacuum too. With a mild street cam, or stock, in your case you should be between 14-19" Hg. I tried to discnnect my vacuum advance entirely, because I could pull 20" Hg @ 55mph. I later found out on a trip to PA, and avg about 16 MPG Highway, that I had too much advance while accelerating. I have since reconnected it. My cruising vacuum decreased a little, but my acceleration reading increased. My city MPG remains barely affected, but my highway economy consistenly avgs aroun 24 MPG with my 3 spd and a 3.08 in my Malibu. You are obviously experienced with building so you'll know when your engine is running right. You'll feel it. Every engine runs differently, even if the are the sme engine. It is inconsistency on the manufacturing process. We don't live in a perfect world.  
Guzzling gas and hauling ass, the true American way.

Make sure that the vacuum advance is connected to the timed vacuum port and not a full time vacuum port.  
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The problem is solved. I replaced the distributor with an adjustable vacuum advance. The pinging is gone.