Have you researched side branch resonator / Helmholtz chambers / "J" pipes? It may be a less expensive way to "tune" your exhaust.
http://penn.freeservers.com/jeep/wj/qfrexhaust/
https://ls1tech.com/forums/cadillac-...ltz-tubes.html
I know it's not a muffler, but it may be worth looking into.
Also, you may want to look at:
