had no problem spinning to 6700 in 2nd in 318, same cam and springs and cam. never knew about double spring recommendation, i thought 901s were good to about 490 lift. definitely full throttle
They are borderline pressure wise. That cam should have about 120# of seat pressure. They probably work pretty good but aren't ideal especially for 6700rpm.
The 901's have .490 is the rated max lift. They are rated at 101#'s of seat pressure at 1.65" and have a 353# spring rate.
Comp actually recommends them for their XE268 cam but their tech line recommends 120#'s of seat pressure for use over 6000rpm. For use over 6000rpm they recommended the same springs that their XE274 & PP280H(their version of a 280 purple shaft) cam would use which are 995 dual springs which have about a 125# seat load and a 402# spring rate or one of their beehive springs. The PP280cam has 280/287 degrees of duration with .474" of lift.
Did you check the installed height of the springs when you installed them?
I just thought I would mention it, but odds are your carb is lean at WOT. That is probably the most likely cause.