are you wanting to do propane for the same reason most guys want it? that is to avoid the bogging/flooding of a carb in the hills? i used to work for a towing company that had a truck that had a 454 converted to propane. they swapped one of them for their longer distance jobs to save $$, the way it worked was the fuel economy was better (dollars per miles), but the power was a joke. you could turn up the propane to a "richer" setting to get the same power as on gas but then the economy went waaaaay down to costing more dollars per miles than the gas did
if you want something that will run at any angle do the tbi swap to your 2.8, propane will end up costing you more money in fuel than gas if you set it right because i don't think a 2.8 could stand to have any less power than it already does