I'm not 100% up on Cali smog rules, but I would look into what ECM would be needed to be smogged as the '1987. From what I understand you would likely need to use the '87 ECM to remain in that criteria for smogging.

This presents a couple avenues. You could use the '87 TBI system, and then use a second PCM or trans controller for the 4L80E later on when you swap that trans in.
This is a cumbersome way to do it, but may be the best way in light of the specifics of this particular situation, due to Republik of Kalifornia smog rules.

Moving to the newer PCM, which as a tuner/hobbiest/hot rodder would be my preferred method, due to the simpler overall set-up and more integrated controller, among other benefits, might change the smog standards that you need to meet. You will need to look into this before getting too deep into this project.

As far as the belt system, that could be set-up with any combination of belts that you want to, as long as the controls are all intact. Personally I prefer serpentine systems, because they are simple and last longer than V-belts systems in my experience. They are less prone to problems as well, such as belt slipping and wear.

So basically you need to talk to your referee, and see what the rules are explicitly and what changes to the system, namely the controller side of things will due for smog standards.