How much would it effect cooling putting the IC behind the radiator?

Would it hurt power or just reduce it?

Also doing a MPFI upgrade, will the 88-91 si ECU work and will it give me any trouble?

 

well you would definitely be restricting air flow by a lot and defeting its purpose of cooling air down to its best ability..

Now how much of a temp difference you have if your running it infront or behind the radiator your having I don't know but it would surely be at least 20+deg.

 

Probally more, the air coming off the rad gets pretty hot.