360 header gains?

My combo is very similar to yours except one size smaller cam, and my car has full-length 3" duals
When I installed my GearVendor overdrive unit, I had to cut the H out of there for room. Afterwards, rightaway I noticed a very significant loss of low-rpm torque. I tried every trick I could think of to get it back but no way.
My combo has a 4 speed, so what it took was a swap from the 2.66low trans to the one with the 3.09 low. By the math, that is a difference of 16%.
Now, I can't say the loss was 16%, only that the new low-gear felt a bunch closer to what I had before the loss of the H.
Obviously, if you have an automatic with a 3000 or more stall, this means nothing to your combo.

I agree with others.
Plus if it makes you feel better, by the time the exhaust gets to the tailpipes, it is a whole bunch cooler than it was at the head. So the gasses have condensed and now physically occupy less space, so the tailpipes can be significantly smaller than the headpipes.
A 3-pass muffler will suck a lotta heat out of the exhaust.
I wouldn't change a thing in your combo, unless your carb is sucking hot under-hood air. Fresh air, on mine, made a very noticible change in driveability
The reasons I have 3"full-length duals on my 360, is twofold;
1) they sound absolutely fantastic at WOT on the way to a 7200rpm shift. and
2) I was hoping that, because of the increased radiating area of the system, it would run cooler, which would help to ease back-pressure, which would allow a cam with a smaller overlap period, which would help fuel-economy, without sacrificing power.