Wow... a lot of discussions...
Well, I made my move across the country. I now live 2000 miles away... and my truck's back there in Oregon, and I'm near Chicago. So, I won't see or drive it again for a long time.
In the meantime, I began aggressively re-tuning the engine while towing and discovered it's happy with very, VERY little advance. We're talking 16 - 18 degrees or so at WOT and high rpm (3600). Yeah, it pulls great that way. I towed a big U-Haul trailer on the freeway with it.
But, then my exhaust manifold broke ( something about the exhaust system putting too much twist on the manifold) and then I moved. So, no more running it.
So... No more propane experiments...