Hei conversion troubleshooting

So I have the points back on the car and I'm tuning it getting ready for my big road trip, and I have an interesting (to me at least) issue with tuning. Under light load, with just a hair of throttle (maybe 1/8 to 1/4") the car runs rough and exhaust pops occasionally. The condition is most pronounced when going around 60-70 cruising with a very light throttle and climbing a slight grade. If I unhook and plug the vacuum advance, it goes away.

So I'm thinking its either too much advance/detonation (dont see any signs on the plugs, and its only at that light throttle setting) OR that its a lean condition just in that one throttle position.

I have initial and mechanical what feels perfect to me, pulls good in 100degree weather with 87 octane and no sign of detonation. Initial is probably around 15. Not sure because I don't have a dial-back timing light and the mark disappears under the water pump anything past 12. The vacuum can on the car adds 27 crank degrees at 15 inches Hg.

I hate to run without vac advance, but i also don't want to compromise how well it runs by dialing back my advance.
I'm also not sure how to richen up the carb (edelbrock 500cfm) for very light cruising throttle.