The 327 is just a SBC, I think we don't really know what you mean by it being a "beast". Anyway, a 700r is a great choice. The Lokar floor shifters are really neat. Check that out.
As you may know, the pic you show are not headers, they are manifolds. You really don't need those. You have them already. If they are "better"/bigger, wouldn't they just add to the "beast" effect? Not getting the goal with those? You will gain +/- 1HP with what you plan. Maybe just straighten out the tail pipe design to run out the back. Which will add backpressure bends.
<<<<<<<<<<<< mostly a joke everybody.
"Who draws the crowd and plays so loud"
