I did a lot of lp conversions over the years in pick ups. Mainly big block Chev. and Ford, and I always had good service with Impco kits. IH had those engines with factory lp so I would use their spec for the timing. If your engine has valve rotors I would take then out as propane is a dry fuel. As far as changing valves at the time of the converation, I would hold off until it starts to give trouble. I have run some big blocks chevs over 100,000 miles before I did the heads. If the head has been overhauled since unleaded gas was the norm it may already have better valves and seats which last pretty good. Also use the IH spec for the spark plugs, generally propane engines require a hotter plug. I go along with the 10% loss in power but it makes up for it with the savings in fuels costs.