I've looked at your proposed runner shape and while I can understand your wanting to lengthen the runners for increased torque at low rpm the shape you have planned is not going to work well because you are smaller at the carb end than you are at the cylinder head. This is the exact opposite of what you want. A very simple, overly-crude analogy is to look at a funnel--works good as intended, but worthless used in reverse. The runner should be at its largest cross section as it leaves the common plenum under the carb and gradually taper smaller as it gets closer to the intake valve (this is a very shallow taper, 2 to 4 degrees). Your probable best form is to imagine the factory intake if made out of rubber, hook onto the carb pad, and pull it up 3 to 4 inches. There is no reason to stay as low as the factory intake is unless you don't have the hood clearance to go taller.
I don't recall the name of it, but there is an enthusiast/GMC truck forum that specializes in these engines, you might try asking over at the H.A.M.B. forum.
Last edited by ericnova72; 03-12-2009 at 12:15 AM.