This is a common swap. Some 61's came with a V-8 and a powerglide trans. You will need as complete of a V-8 as possible as none of the 6 cyl parts will interchange. Front & rear aftermarket crossmembers are available for mounting. You will likely need to modify the driveshaft and replace the front yoke. A complete exhaust system and a shift mechanism are also needed. Some electrical mods are required and you may want to upgrade to an alternator. You will probably need to upgrade the cooling system as needed for an automatic trans. It just takes some knowledge, cash, and work.
Fred


1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine