Gabe,
How do you propose your going to swap manifolds? You talking swapping heads with your stock heads as well? Thats a TON of work in the name of science but would give a good indication of the different top end set ups.
I just gave this to Eloy, its a simple flow calculator you can use to see how much the rail will flow, its very simple doesn't account for fluid viscosity, friction, eddies, etc. But shows that at 500 lb/hr of fuel you need a 1" diamter pipe ie rail to support. What fuel pumps are you going to be running?
http://www.1728.com/flowrate.htm 
