our 3,9 runs a bit hot too if the rad is blocked outside with mud or inside with sludge. If you're happy the guage is ok (as Tim says) then take out the rad and clean it inside and out, maybe put a Radflush set through it first. You can get a wetting agent from motorsport places like Merlin motorsport but you shouldn't need it.
Have you had the heads skimmed to much, what's your timing set at etc etc, basically is it re-tuned for the LPG?