Trying to did out if a exhaust from a 98 ls stock exhaust will fit onto a 94 rs the whole exhaust from the headers back?

 

Yes, it should be a direct bolt-on. The 94-96 LS and RS along with the 97-01 LS, RS, and GS all had the same exhaust systems when they were produced. You should have no problems mounting the cat-back exhaust system in the way you described

 

it should fit but since the 94 rs is obd 1 and yours is obd2 the o2 sensor should be on the cat instead of the headers other than that they are pretty much the same