It's perfectly fine to run an obd1 (or 2) engine with obd0. I have an obd2 b16b in mine running with obd0. Got 201 hp last rolling road at 8k. If you have the throttle from the b18 with the map sensor on it, I'd use that. The sensor will then be sensing the correct pressure whereas if you are using the firewall map sensor and plumbing it into the b18 intake manifold, it won't. It will be near enough but if you wanted to be pedantic....
There is also supposedly a difference between the two map sensors. The obd 0 measures 1.7 bar and teh obd2 one measures 1.8 bar. I have also heard that the technical specs show this not to be the case but I haven't seen them so can't confirm.