As far as floor pan clearance I can't help you there, never owned a 1st gen. I think there is plenty of room, but I don't know for sure.
As far as trans swap, car aluminum Powerglide and TH-350 short tail(6" tail housing, 27-5/8" from case face to end of tailshaft housing.) are the same length, have the trans mount in the same location, use the same crossmember, use the same length driveshaft and the same slip yoke spline.
You'll need different shifter linkage, and a slight tweak to the cooler line angles, and dipstick's are trans specific and don't interchange.
If the TH-350 is a stock style automatic shift build with a kickdown cable, you'll need the cable bracket for the intake manifold for the cable.