All the 12v needs is battery power to the starter and a signal wire from key switch to selanoid on stater. Both wires should be there from the original motor. If this is a ve pump then it needs a fuel shutoff selanoid positive feed to turn the fuel on. Also could be a neutral safety switch issue.
near as i can tell from my pile of wiring diagrams the park circuit is a bark blue with orange stripe and tan with red wires it appears you need to hook the dark blue with orange stripe wire up to the center wire on your park switch on the trans..the tan with red appears to ground at starter relay..
if you need power I am planning on using keyed hot relay for the injector driver module for fuel soleniod...I'm also using the dodge engine and trans in my swap
on edit..I went out and looked at my 550 and the tan and red wire goes to energize the start relay so may need to jump them together to get it to crank up if your having that problem..