I have a Headache.... Need some insight into Exhaust...

This will be a interesting thread. Looks like you've been doing your homework. The Spitfires from what I understand flow one step better than 340 manifolds although I have never seen any dyno results. They also exit right over the torsion bars and the exhaust pipe is a pretty tricky design. May be someone has a set of Dougs that you can try with the o/d tranny. If you are pushing in pressure you need free flow out. (you already knew that)