This is my own experience of running LPG; make whatever you like of it.
I had my 3.9 LPG adapted at 75k miles by an recognised convertor. 10k miles later the engine developed a light tapping at
piston speed the moment it reached running temperature, no noise up to that point so I knew it wasn't the hydraulic tappets. Early on switching back to petrol whilst driving resulted in the 'tap' fading away only to return within a mile or so of switching back to gas. Before long the noise was appearing shortly after starting from cold & really loud. A subsequent strip down revealed #3 liner was moving & I had to source a new block.
Exactly the same thing happened to a mate's 3.9 after roughly the same post-conversion (by a different installer) mileage. Both RR's had enjoyed a perfectly healthy cooling system.
The good 'ole 3.5 may well be ok with the gas because it has more 'meat' around the liner, but IMO any 94mm bore RV8 is vulnerable