I got a chance to get a good deal on the Racing Beat streetable header and presilencer off a friend's 86 RX7, but I own an 84 GSL-SE.

From reading the information off of RB's site, the presilencers are each 20" and the mounting flange presilencers are different. The notable difference I see is that the S4 set is 2.5" vs the GSL-SE 2.0". I am unsure if the other dimensions are much different.

The parts in question:
86-88 NA exhaust parts
Streetable Header for 86-92 RX-7 NT - Racing Beat
Presilencer for 86-88 RX-7 Non-turbo - Man Trans - Racing Beat
vs
84-85 SE exhaust parts
Streetable Header/Collector for 84-85 GSL-SE - Racing Beat
Presilencer for 84-85 RX-7 GSL-SE - Racing Beat

Is the swap possible without too many changes? If so, is 2.5" going to be too large for the engine? Any other things I should worry about?

 

getting it to work shouldn't be too much of an issue. worst case, you buy some flanges, a small section of pipe and gaskets and build something to adapt.

from my own personal experience, i would suggest confirming fitment first though. i have not conclusively confirmed it, but my initial finding is that there is a difference in curvature of the exhaust system between Gen I and Gen II. check to see if they will align.

 

2.5" will be perfect for the engine. I don't know why RB stubbornly clings to 2" for their 1st gen exhausts. 2" hurts power and probably reduces fuel mileage.