I say find a different muffler shop. What engine modification are they talking about. Dual exhaust is the number one bolt-on power upgrade for a V-8 engine or most any engine. If you put a V-8 in your 46, I think that you'll find it easier to have duals made for it rather than trying to have some fancy bends done to make a single fit nice under your truck.
There's my opinion,
Scott
