Felony, Your FT headed towing motor sounds neat, I saw an S.P.2.P ( hope i got that right...... ) on fleabay the other day for like $150.
A performer ( not RPM ) might be a good choice as well.
I always thought the FT exhaust manifolds were WAY better than the stock truck Log offerings. The FT manifolds look a lot like the classic Chevy Rams horn. Gotta deal with the cross over, but thats not a huge deal.
Propane can be hard on valve guides, we had a costumer when i worked up in wyoming, he worked for the propane company, they all got propane for free basically, so most of them had converted their truck to run on it.
He came in about once every 2 months, had several boxes of pushrods behind his seat. We would pull the valve covers off his 460, pull the pushrods out and replace all the bent ones. Usually about half the intakes were bent. It was a pain in the ass for him but he didnt care, he was saving a ton of cash not having to buy gas for that smogged out turd of a 460.