trans swaps..

Here in lay my question,
If i take out my1966 225 slant six.. with 3spd(whats the code for this trans??)and put in my 1972 360 with auto trans(code for this too??) do i need to use the trans that comes with the motor? or can i use my 3spd? where can i get a 4spd? is a torqueflite just any regular autotrans??

Thanks and sorry if its a repetitive question.. id just like to ask now and know for my self instead of reading millions of threads and getting 15 different answers.

Cerwin