Sorry this reply is a month old.
I put duals on an old Olds engine. The crossover inlet pipe to the right side manifold can be capped with a piece of steel and brazed on, or the last couple of inches of exhaust pipe can be cut off, and the stub closed off and welded shut.
This leaves you with both manifolds with ordinary outlets. I found it's best to have a muffler shop with a custom pipe bender make the pipes for you.