Project 51, headers are designed for a universal range of operation. When you start talking equal length pipe, tube size, collector size or length, you are getting in to very specific header design. Each set is custom made for the vehicle they are going on. Yours would be complete different from someone else's. The whole vehicle is looked at and how it will be driven before any designing starts.
For your situation, you would be better off using a free flowing cast iron manifold if all you can get is a shorty header. The manifolds will out last the headers and work equally as well. To gain more performance, you need a long primary tube header with a 2 1/2"- 3" collector. To big of a pipe diameter hurts as much as having to small of a pipe diameter, more length increases torque but can lower peak horsepower. The key is to use a average length and diameter to get a decent power output at all rpm ranges.
I would look in some of the LS-1 sites for information. The guy's running them are make tons of power with few modification and could probably steer you in the right direction.
Joe
