Are you running the 305 exhaust manifolds or the 350 manifolds?
I would think the two Y pipes are completely different as the exhaust manifolds are quite a bit different as to where the outlet end sits exactly in relation to the block. Dipsticks are on opposite sides as well, so that changes the manifold shapes a little as well.
I would highly recommend taking it to an exhaust shop and doing away with the Y pipe and switching to duals anyway. That will let your motor open up and breathe so much better.