Depends what year model 760 you have.
Until year model 88, the electrical system were substantially the same as the 740. From YM88 on, they were very different - fuse box in a different place, etc.
Wiring diagrams were on a Russian auto electrics site a while ago, but it's said that Volvo gets a bit cross about their wiring diagrams being placed online, and so that site may have gone.
To get legit wiring diagrams, there are USB sticks available with all the service manuals on them for about $100 OZ pesos delivered. They have ALL the info on the car.
https://store.otpubs.com/740-760-780-models-1982-to-1992/
Expensive? IMO not really, it's the price of 2 meals and a couple of drinks at the pub these days.
Best transplant into a 760 (IMO) begins with a B6304 from a dead 960, and gradually built up with twin turbos sourced from a dead S80.
The 253 lump thankfully can't be put into most 760s legally, since most 760s (1987+) were ULP only and the 253 died with the VH Commodore. Gawd knows why anyone would bother with that turd when the old Buick V6 from the VN Commodore made more power and torque out of the box than it. Even before the V6's exhaust is de-restricted and before tinkering with the ECM Memcal, doing the ACL bearing stuff, easy head work, etc. The 308 is a similar story insofar as ULP goes.
The LS1 is superior to the 308 (304/4.9) in every way - aftermarket parts availability, $ spent v power made, power and torque, fuel economy v power - if you're hell bent on doing that, and Yoshifab et al have kits.
