Sounds like something may be wrong...
I have a "all on" headlight also, only it's not the cheap stuff some are selling now. Mine is a complete wiring harness with 2 relays that are connected to the battery not the light switch like OEM or some other harnesses.
I can and have run all lights...
Did you use a scanner to time it after installing the IP? You can't just install and go, it doesn't work that way ..
Edit... I see you didn't replace the IP, might still be a good idea to plug in and see what is really going on.
Personally, I think you might need to except you have a 30+ year old motor, weak rings, slick cylinder walls, IP wore, among other things, until you go through the inside of the motor not much is going to change... Such is life...
And the way the flange surface is machined doesn't make things easy either...
I think I'll need to do a little machining on my heads to make them fit better..
I've contacted the shop that produces those flanges, they won't make them in ss... So I'll play on cad and make my own, I'd like to use 1/2" for the flanges and transaction from rectangle to round on the flanges, but we will see, I'm not to good at cad...