I'm about ready for initial start-up/breakin on my rebuilt engine.
Can I just wire up the engine harness to the HEI, starter and battery cables or do I also need the horn relay and voltage regulator to start the engine?
Some others will chime in but the main ones you need if I remember correctly are: distributor, coil (or HEI power), starter and wiper motor. The wiper motor has part of the starting circuit. At least mine didn't work until I did that. Voltage regulator isn't as important as long as you got a new distributor.
Make sure you got good grounds.
Its been a while though and don't have schematics so not 100% positive. Some others will chime in.
You will have to use both the engine and lighting front harness. The power to the HEI and and the starter (neutral switch) and fuse bock come from the lighting harness. You will have to have the alt wired (so you do not have a live 12V wire out there), horn relay, and regulator also. That should do it to start it up and break in your new motor. Good Luck Jerry
68 Red with Red & white interior, 383, Offenhauser Crossram, 700R4, Ocala FL