Talked to Rob at C&S Perf and he said the TKO600 2nd to 3rd shift was a concern a couple years ago but Tremac put a fix in that modified the rail which cured the problem. As far as the dual plane vs single plane Rob said it's the height of the single plane that is the issue in Cobras.
I did find this on Edelbrock's site however;
VICTOR FE (4000-8000 rpm)
Designed for high performance 390-406-410-427-428 c.i.d. Ford FE engines. The singleplane design provides race-winning power for many types of competition. Carb flange height is equal to the Ford high-riser manifold #C4AE-9424-G to fit under 427 Cobra hoods. Runners have plenty of material for port matching to low- or medium-riser style heads. Machined rear breather boss is included with an unmachined nitrous bosses and front
oil fill tube boss. Will not fit 427 high-riser and tunnel-port heads. Match with our FE heads. Victor FE also available with injector bosses.