Ford 5.0 with impco 425

[Mark_efv8]

G'day all
I recently bought an impco 425 mixer to replace the 225 that's running on my ef 5.0. The gas setup was running fine but a bit under powered for my liking so I've decided to upgrade. I've recently upgraded the motor with gt40p heads cobra style xr8 manifold, extractors and 3 inch cat back exhaust and in the process I also changed the spark plugs. Now since then the car has been idling a bit rough. I put in a set of bosch iridium platinums with the multiple points on them and I have reason to think they might be causing the rough idle. I'm gonna get the gas joint to install the mixer so I'm thinking of getting them to change the spark plugs to LPG specific ones with the appropriate gap. I also want to set the base timing at some point as I was having trouble with pinging on petrol above 4 grand. Any thoughts on these points would be much appreciated.
Cheers
Mark