72 GMC Jimmy engine/transmission suggestions

[87Skier]

I got this from my father-in-law. He said he was hauling to the junkyard if I didn't take. He really had to twist my arm, if you know what I mean.

There are some good and bad things about it. First the good is that it now runs and drives (it sat for 17 years without driving, but after changing some fluids, new cap and rotor, carb rebuild, new gas it runs pretty good), second, "light" rust, third, everything is there except the top.

Now the bad, the top is missing, the rockers, floorboards and some of the supports are rotted out, and the engine is not original.

Which brings me to my next point, I can't say I'm a fan of the Chevy 400 motor, and want to switch it to the stock motor, the 350. But, I want to keep it mostly original, but make it more drive-able at the same time, so should I swap in a carb 350 with a 700r4, 4L60 or 4L80 behind it?

Also, I'm planning on doing a body off restoration in the future, but not for a year or so.

Forgot to mention, I don't hate the Chevy 400, but this one was a hack-job. Some one made a custom alternator mount, string is required to keep the radiator hose off the cooling fan, and the water pump/cooling fan are misaligned with the crank.