Voltage regulator meltdown/70-amp Powermonster.

ouch! nothing worse then your issue. well seeing that you changed engines how about the grounding cables?
one from engine to firewall
one from engine to frame
one from battery to block or head?
good grd where voltage regulator mounts to firewall?
any added electrical items like a on/off moroso switch for battery?
keep thinking you will find it
terry