The same 89 IROC-Z restoring for my grandson. Going to install manual oil pressure guage. Unsure where the sending unit is. The engine is (?year) a 350 bored to 355. There is a sending unit just to the left(drivers side) of the distributor, is this it? Thank you!

 

Yes, the port by the distributor is the best place to connect to. The factory plug can be a real pain to get out, but luckily yours has already been removed. If your block is the same vintage as the car, then the original oil sensor location is down near the oil filter, above the boss.