The power glide does not have a torque converter clutch so it won't cause the stall. If your stall is still happening in neutral then you can rule out the tranny since no clutches will be involved.
If you have capped all your vacuum sources and still have the stall I would certainly look closer at an electrical problem causing it under heavy braking.
Good luck. Hope you find it soon. And get the vacuum advance hooked up to ported vacuum.
