I’d try opening up the lash .004-.005 and see if that impacted the 60’ time
For perspective on the stall speed....... the Normal T/A 8” Race converter for a 440 bracket race engine would flash about 5600.
That configuration was derived long before there were Indy heads.......... so it was lotsa stall....... with small heads.
Typically, the trend would be to increase the stall speed as the heads got bigger.
 After watching the video, I think the converter is the main reason the 60' is lazy. If that's the case, adding some lash should help the 60'. I'd do the change back to back at the track to rule out day to day changes. 
CHIP
'70 hemicuda, 575" Hemi, 727, Dana 60
'69 road runner, 440-6, 18 spline 4 speed, Dana 60
'71 Demon, 340, low gear 904, 8.75
'73 Chrysler New Yorker, 440, 727, 8.75
'90 Chevy 454SS Silverado, 476" BBC, TH400, 14 bolt
'06 GMC 2500HD LBZ Duramax
| 
