Check the distributor. The mechanical advance in any distributor must be checked on occasion, in time they get junk and crap on them which makes them stick. If the advance sticks open and you go to fire it off it could be in full advance which will cause it to kick back. pop the cap and grab the rotor, If it will move then see if you feel any sticky or rough spots. If you do then inspect it and find out why and where. In most cases if it's severe you'll have to remove the distributor and disassemble the unit. Before I use any HEI the first thing I do is strip it down then clean everything on the mechanical advance. Then I use fine grit sandpaper and polish everything that operates with the mechanical advance as I want it to operate smooth as glass.
If this proves to be the problem then fix it then move on to replacing the balancer and the problems should be over.