Nice that Bayer Brothers is still there. I used to visit them in the 1990's when they were in Mableton. As 64drtGt suggests, you need an engine that matches your transmission. Around 1968, the tranny's torque converter snout got bigger, as did the hole in the end of the crankshaft. If mating a later engine to an early tranny, you need a special $20 adapter ring to take up the gap. A member here sells them for $20 (Charrlie_S or such). The other combination doesn't work.
The hardest part will probably be removing the top bell-housing bolts. When I did it long ago, I didn't have the best tools. Today, I might use real long socket extenders and swivels to go over the tranny and use a socket wrench at the rear. I think you will need to install the tranny cooling tubes before you bolt on the driver's engine mount, since they go over the mount (all years?). Installing the engine takes <1 hr with a good lift. You may spend several days hooking up everything else, especially if you do it right and clean, paint, test. Hope you aren't rushed.
Remove the radiator for more room, and not to damage it. Now is a good time to remove the block drain (3/8" NPT plug, passenger side) and flush the block. I put a GM knock sensor in my drain hole for future use in spark control.