91 RS was a 305 tbi did a 350 tpi swap from a 88 it is a speed density swap as well , I am trying to sort out some small details it's running on it's own pretty good thought it had a high idle 1500 but I think the tach is off it might actually be more close to normal , anyways sometimes it surges rpms go up on there own and down , getting a code 33 and 42.I did notice the map sensor has a vacuum port on it is this spose to plug into throttle body? Mine is from a 88 maf car I don't see anymore ports to use?

Map sensor port

 

Egr solenoid went to the tb. The map went to one of 3 port on the p/s of the plenum. Looking at it, your plenum looks to only have 2. Originally a maf plenum?
edit, scratch that last question, I just reread your post. Yes, being 88 it was maf, you'll need to add a port to maintain everything the factory has. I had a 89 plenum that I added the port.

the original SD plenum also has more material and a bolt hole to attach the map sensor bracket

 

Oh ok, I was suspecting that wanted to verify, thanks so much!