i dont know off the top of my head if itwas longer or shorter, but i'm pretty sure the gsr cable was longer because the way the GSR intake manifold is designed.

as for your backfiring, did you make sure the timing belt was lined up correctly in relation to the crank? its a long shot, but u wont know for sure unless you check. where did u source your b18c1 from? swaps always have mystery history's so you have to make sure all your base are belong to us, i mean all your bases are covered.