100 cfm more with anular and downleg boosters can't beat AFR

Something you can try to do if you have access to Avgas you can run it on that as it has a lower point at which it vaporizes and so if you see an improvement then you know that's what you need to work on.

Or you can change the cam or the carb. The carbs easier to change and I would either try a 600 with straight boosters or get the 650 boosters changed. My preference would be the 600 as the it would increase the vaporization more so than the 650. Main bodies are cheap so you can use everything else. Its funny how on a running engine reversion can make its way all the way to the underside of the air cleaner. If the incoming charge isn't sufficiently vaporized in the intake runner then there's very little to oppose the exhaust making its way up. Do you have a smaller intake manifold you can use?

Like I said earlier advancing the cam will build more cylinder pressure which is most probably what you need.