[John Owen]

Just a brief update - I changed to the SCJ heads, the HP profile is almost identical in shape at rpm, but adds a little HP at low revs and now peaks at 553 RWHP, or about 660 at the flywheel. Single disk clutch started slipping badly, so I changed to a dual-disk, works great and holds.
I also compared breathers while at the dyno. The homemade box produces 25 HP more than the Ford oval with Ford filter.
As to the flat HP peak, that is a great benefit of big motors. Like Lubrecon said, you don't shift on a dyno, all 4th gear, but when the time comes - Shift at 6500, if gearing drops the revs 1700, you still have near peak HP. Fastest acceleration is achieved by maintaining the highest average HP to the wheels, within the limits of traction.
Hope the information is useful.