Dont believe everything about electric fans.. If going to replace a fixed fan.. yes.. If a clutch fan.. no.. forget the marketing propaganda to sell electric fans (like the 'flexi fan bs) .. . there is no load on a clutch fan when it is not needed.. this is one of those old wives tails.. like HEI was a upgrade, in the early 70s.. it wasnt.. it was introduce will the introduction of EGR.. lean cruise burns, and lean mixture means more resistance across the spark gap that need more energy available. to meet that extra drawn
lower compression.. cylinder pressure... means less advance higher octane means more advance needed.
So taking a low compression medium range/ duration rpm cam engine engine one hand you need less because of low cylinder pressure, but since LPG has a octane (NZ, most of the world rating system) of around 100, and std petrol only 91.
Also if want to increase cylinder pressure , decrease the duration of the cam for better low end cylinder pressure.
Which as I state before, compression ratio means squat..
it is the cylinder pressure thats important at a given load and rpm to meet the octane requires of the supplied fuel.. cam duration and overlap (little simplified) is the time the both valves are open on the compression stroke..the amount of pressure that is lost before both valves have closed (time this are open is also determined by the rpms... so higher the rpms les time to leak, higher cylinder pressure... So want bottom end torque/ hp low duration low overlap.
A classic extreme example is WW2 (1936 +1954) British 'Pool" fuel had a very low octane.. to meet this octane British engines where meant to have a compression ratio around 6.8 to 7 :1.. in reality they had around 5 to 6:1 and used a 'square' cam.. ie one that had no overlap that had any 'leak' on the compression stroke.
I also went two steps heavier on the advance springs that on paper have everything in (with the black bush) at around 2500 rpm. However this does move the start point of the centrifugal well after idle.
The issue you have is you still get a slow curve rate with the bush.. go to using stops as I described above, and light springs.. basically you go as light as possible but not light enough for the springs to bounce at idle or fast(choked) idle. If you do, the advance increases, the rpms increase to a point the idle mixture circuits cant supply the required fuel, the rpms drop and repeats. you will find that usually cent all in at 2500 rpms will bounce the weights.. 2600 pushing it, 2700 good.
So timing aside I reckon I still have something a little weird going on (some sort of restriction?).
If was a 425 I would say remove the centre piston and check it and the bore for the most tiny of machining burrs.. a very light rub with around 1600or 1800 wet dry . Many yrs since messed with a 300...I would guess a similar situation inside that.