One of the advantages of buying a whole vehicle to get the bits for a conversion is that you have plenty to choose from. I used the fuse box from my donor car, so things like the fans(from the donor car as well as the radiator), and fuel pump relays were all there with the wiring ready to go. I just basically wired the ignition wiring from the PCM straight to my trucks ignition switch. It wasnt as easy as it sounds, but not that bad.
So, I suppose what I am saying is, get as much gear you can from the engine vehicle.
Just makes it a lot easier in the long run.
And I nearly gave up a few times as well. Just take a break for a while if it gets to you.
1978 C20 short bed pickup, GEN111, 4L60E trans.