I've got a LS right now. I've decided forced induction just isn't a smart thing for me and I have no reason for it. I only have one car, so I need to keep it reliable. I figure I could do some basic bolt ons to my LS for the time being, and when I've got some more money, do a B18C swap. My question is though, if I do the bolt ons to my LS, how much compatibility am I gonna have if I got a JDM GSR? I figure the intake wouldn't be, because of the difference in IM's, but would everything from the header back be? I know with a USDM 00-01 GSR, the exhaust is the same as a 94-01 LS, but I don't know about the header and cat. Now if I got a pre-00 GSR motor, I imagine I might run into some problems. I just wanna know that if I buy parts now, they will be compatible later on. If they won't be, I'll wait til I swap to buy things. I don't have an nice paying job, so I'd like to keep good gas mileage but still fun to drive and stock LS just isn't fun. I figure a mild NA setup on a GSR would still get me fine gas mileage and would be a big improvement over an LS.

 

First off, everything in the LS head would be worthless, and the Intake Manifold itself is no good...however the injectors, sensors, throttle body, Intake tube and header/cat-back would still be good.
As for the bottom end, aftermarket flywheels, pullies, and oil pans are compatible.
Be careful of mixing internals because the B16/B17/B18/B20 use slightly wider rod bearings than the GSR's and ITR's. This means that any aftermarket LS forged rods without machining could cause big problems on the stock GSR/ITR crank. Now, if you are going to stroke that GSR with the LS 89mm crank, then you can use any forged rods or pistons from the B16/B17/B18 & B20.

Did I miss anything?

 

The only parts I was really concerned with are the I/H/C/E. I wouldn't start working on internals until I swapped. Clutch and flywheels also matter since I may need to buy a new clutch in a few thousand miles. The extra info was helpful though, thanks.