Joe Cole, The path of least resistance would be finding a 56 or later 235. There are still lots around and many good ones got pulled in favor of small block V8s or 261s. The water pump location on the 55 2nd and later 235 isn't insurmountable and on a GMC you would have enough room for the 235 fan. On Chevs and probably GMCs, you need to drop the radiator in its mount a couple of inches so the lower hose clears the fan unless you use the available modified water pump mounting. But you can get by without that, I have for many years.
A 235 will bolt to your bell housing and so use the stock transmission. Parts for a 235 are cheaper and more readily available than for a GMC with the exception of camshafts blanks and for the 55 2nd and earlier 235, main bearings. So I would look for a 56 or later 235, Powerglide or stick, doesn't matter. Aside from moving the front motor mount back a couple of inches and the water pump, its a drop in .
