Keep an eye out on CL or check out the Long Beach or Pomona Swap Meets.
I have seen them installed in many older cars. But I didn't ask where they purchased the engine from. I know you will most likely need a stand alone computer for the ignition.
The one guy I ran into, had installed a newer Vette engine in a 71 Z28, along with a six speed. He was getting his engines from a Vette shop. The shop was pulling the engines from wrecked late model Vettes.
As he told me, one of the engines he purchased, ended up being junk as it had a cracked aluminum block. The shop honored their claim that the motors are good and replace the motor with another one. So I would really check the condition of the engine and get in writting that the engine is not a piece of aluminum junk and will be replace if not rebuildable.
One more item, he had to get some custom headers made for the engine/car combo.
I know other people on this site had done the conversion, I would ask a lot of questions before starting this kind of swap.
Good luck, sounds like a cool idea.