Just finished putting in a 5.3L from an '03 Suburban in my '73 Landcruiser. Got the wiring harness from Nelson Performance and probably spent about 30 minutes connecting the harness to the motor. Cycled the key twice and it fired up on the first try!
I will get some pics and post them soon.
I debated on trying to build my own harness or who to go with - totally glad I decided to give Nelson a call.