Exhaust in 66 abody

You're definitely on your own. I don't know of any off-the-shelf header to put an RB engine in an early A, much less an underchassis header. A 440 into an early car has been done, but it's not a popular swap for a reason; it really doesn't fit well.

Have you considered flipping C-body manifolds upside down with short stacks through the hood?