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1956 GMC want to upgrade to 327/400 automatic trans.
what effects to rearend need to be taken into account,to keep her planted on the ground.if any

nothing wrong with using the original rear unless you're planning on mega horse power and lots of hole shots, the 56 was originally available with a V8 .... you'll be looking at some mount fabrication to install that combo, and of course rigging some type of shift setup .... you might determine the actual rear ratio and investigate the possibility of an overdrive automatic, like a 700R4 if you expect to see much highway time

Bill

I would consider using a 700R4 with the stock 3.90 gears. You would get a better launch and a better cruising speed.

I destroyed the factory Eaton HO33 Rear in my 59 gmc 1/2 ton long bed when I swapped motors. The cast banjo housing could not stand up to the torque of the engine I was running and was literally ripped apart. The engine I was running was built for about 325HP and 450/lbs torque.

I believe the factory rear should stand up well to 350 lbs and less.

thanks guys
i think i will take your advise and install 700r4 3.90 gear rear end/& floor automatic shiftstick