Just use another 4100 it is only about 1500 bucks and if you take care of it you wont have any other problems . I put a 307 Olds in mine , the trans didnt work i had to get a new one and i had to change the motor mounts.When it blew rp i put a 350 chevy in it I used the mounts off of a 90-93 fleetwood ,youll have alot of wires to cut and reroute , and with the 350 or 307 you get about the same power as with the 4100. But if you really want the swap you can use a Chevy small block , Olds 307-455,Buick V8,or a pontiac V8.There are mounting plates that make any of these engines work with the original trans, summit sells them for about 70 bucks.You could get a 4.1 Cadilac V6 from an 81 coupe from Autozone for $1200.One of the engines above will work with the stock trans but Im not going to speak on what I dont know [remember I changed mine] An older 368-425 Cadillac motor will also work.Or just get a whole engine and trans out of an 86-96 cadillac fleetwood , they had 307 Olds and 305-350 Chevy engines
I got a nice ass 4100 for sale with the trans. has only 40000 miles on it. You wont find a better 4100 for sale. make me an offer, I will go ahead and answer you question as to why I am not using it, I am building my car radical custom, and the 4100 just dont look right in a radical custom car.