2.5" in to 3" Y-Pipe
Don't buy the electric cut out and you will have more money for a real exhaust.
If you are cash strapped run 2 1/4" dual off the headers (to the mufflers and don't buy expensive mufflers) then dump it at the rear axle.
If your budget allows go with 2 1/2" (but 2 1/4 should be fine for a mild 360)
Then……. When you have some more money, put tail pipes on it.
Sometime along the line when your budget allows have an "H" pipe added.
There…… You are done.