I agree, I would be looking for sufficent clutch releasing. I have had several Toploaders, never had one that i couldn`t powershift at WOT, providing the shifter was adjusted properly, and the clutch was releasing cleanly. That said, I rarely twisted any of my Toploader cars over 6500 RPM, but never had any issues at 6000, except once on my 70 R code Mach 1, when, unbeknownst to me, the Zbar started to tear at a weld, and reduced the clutch airgap. But once I fixed that, no more issues.