I have a 91 tbi car that i put a 87 350tpi in. Will a single fan do a good job keeping it cool? Basically which is better dual or single?

Dual fans serve the purpose of cooling in 'stages' essentially (unless you rig them to run together at the same time), but they come stock to turn on at specific temperatures. The stock dual fan setup on the thirdgens is lacking though. Not sure how the TBI fan cools, someone else can chime in on that one.

You can compare the cooling effectiveness of a single vs. dual setup by comparing CFM ratings. With dual fans, you have to add the CFM that the fans pull and compare it to the CFM that the single fan pulls assuming that both fans are running of course. So if a single fan moves 2000CFM and your dual TPI ones pull 675 each(I think that's about right), well then you are better off with the single.

Also note, that the surface area that each fan covers is a factor too. If you get a fan setup with a shroud that covers the majority of the radiator, then you can expect it to cool MUCH better than a fan setup with no shroud. Take this into consideration too when making your decision.