The fact it runs good on petrol leads me to think ignition side isn't the problem. If ignition was the problem it wouldn't run good up top on either fuel.
Jaycar make these kits they're pretty accurate i have compared them with my $3k digitial air fuel ratio meter . I sold my AF meter while i could still get good money for it when people realise how good these Jaycar products are they would never spend heaps on a professional meter. The LED light kits are even cheaper the ones they were selling 30 years ago were really good.
https://www.jaycar.com.au/fuel-air-mixture-display-kit/p/KC5485I would look at A/F ratios on Gas to see what is happening. Not sure what type of LPG setup you have i know the Gas Research LPG carby was really good worked much better up top than other systems.
The fact you have vacum advance tells me this isn't a fuel injected vehicle. Mechanical /vacum advance systems belong in a museum.
Back in the early 90s i converted my 265 Hemi powered UC Torana hatch to fuel injection. Had to weld up mec advance piss off the vac advance used a Lumenition optical pickup to replace points and stuck a MSD 6A on it running through Haltech ECU. The MSD is under brake booster.
The gains on ignition side were massive at part throttle being able to map it through entire rev range at all load points. It always had a flat spot around 2% throttle 2000 rpm with triple webers. Cranked ignition advance up to 38 deg at that load point flat spot was gone it purred like a kitten.
It's the old saying if you want it done right sometimes you have to do it yourself.
Gas will allow more ignition advance so you will never tune it right to run on both fuels with mech/vac advance. To get best performance on gas ingnition timing will be wrong for petrol. One fuel with have to be compromised for performance with ancient advance controls. If you have a switchable ignition map it can be tuned to run great on both fuels.