Cool. Congrats on being creative. The last time I did something like that, it was '78, I was in high school, and the car was a V6 Capri that I put an H pipe into and then merged the duals into a single race muffler at the rear. It was fine below 3000 rpm and ear piercing over 3000 rpm.
I had a buddy do the welding, though. If I had welded it, it would have weighed twice as much and fallen out on the freeway.