if your ignition system is in good shape a HEI conversion is a complete waste of money
that said you are having problems, i would diagnose the problem because you can waste money on replacement parts only to cover up what the original problem was or is..
the car is misfiring.
so first inspect the ignition system for loose wires, or wires that have the insulation damaged. is everything tight for the distributor cap.
inspect the cap and rotor, pull the wires and pull the plugs have a look at them, is there a mechanical issue, improper gapping?
if you are using points inspect the switch and possibly replace the condenser as a possible problem source.
some people have really worn out distributors and they are using points. there is a cam on the distributor that can be worn out and cause the points to fire differently on each cylinder that can lead to misfire, an electronic conversion would not use the cam for timing and that can solve a problem, changing to HEI would solve it as well but at a much higher cost.
possibly replace the coil with another one to see what effect it has.
aftermarket ignition wires can cause all kinds of misfire/crossfire and interference problems, OEM wires are great and work well even in most racing applications.
you can do a ignition system tune up if you want to rule out existing parts as the problem, replace the plugs, wires, rotor, cap, points, coil with new and see if the problem is still there.(it might point to another issue)
check to make sure the ignition wires are neat for cross fire and the firing order is correct.
you might have another problem and spending money on a HEI will not fix it. a petronixs conversion works great and keeps the stock look and parts.
we are looking at this from a ignition problem. but you could have a valve problem or head gasket leak or a few other things.