My 408 combo is
Scat forged stroker kit, KB744 pistons,Scat forged I beam rods and forged crank
Eddy RPM heads ported and no flow numbers with K800 springs and 1.6 ratio PRW roller rockers
Comp XR286R cam with Howards lifters (no need for oiling mods)
Super Victor Intake and Holley Ultra 850hp
TTI headers
MSD digital 6 and locked dizzy
904 trans,low gear, 4800 J&K converter
4.30 gear with 9" Hoosiers
3420# with driver

When I first built this motor with the 590 cam and had the engine on the dyno it pulled 537hp 540 tq with 750hp carb and TTI headers
We tried a 830hp carb and 1 7/8 headers and it pulled 550/550
The more I think about it the more I think the issue may be in the trans?