Are there differences between the 670 H.O. and standard 670 ignitions? The H.O. is set at .126 btdc and non H.O. 670's are set around .076 btdc. Could the flat top pistons in the H.O. be a factor?

The timming advance could have some thing to do with the shorter exaust track (y-pipe) there may even be some small port timming changes(exaust) All of this moves the peek HP rpm up to the 8100 vs. 7800 on the non-HO.IMO
JIM

The keyway on the crank is also positioned differently.