Welcome to the forum!
I am assuming by your description that you have an ex Schwann's truck? I have one as well, it is a 6500 with the same 8.1. I bough mine specifically because it was propane powered, as our family business was propane for most of my life so it was an easy choice for me. Plus, at the time I built the truck propane was about $2 per gallon and gasoline was about $3.50. Now, it is the other way around with propane at Flying J at about $3.29 and gasoline back to $2, so I feel your pain. If you are mostly driving local, you need to do as I have and find a local welding supply that will give you a good discount in exchange for regular business. My guy is about 2 miles from my shop and fills me for about $1.50 per gallon and the tanks are big enough that generally I don't have to fill anywhere else. The exception is a long trip, and those Flying J's get expensive.
I am also assuming you have the BiPhase system on your truck that Schwann's was using on that year. My understanding is that BiPhase used all the oem electronics and wiring with the exception of the Schwann's add on harness which plugged into all the oem connectors. You can generally tell the Schwann's harness from oem as it is a cloth covered as opposed to the oem tape and wire loom harnesses. It is my understanding from talking to folks who have done the conversion back to gasoline is that you will need the following from a gas powered truck:
fuel rails and fuel injectors
gasoline fuel tank with appropriate in-tank fuel pump
fuel lines to connect tank(s) to the injectors
All that will plug directly into the existing (oem) harnesses and use all the existing electronics, just eliminate the Schwann's harness. If you have a dual tank truck, the gasoline setup will use the same as the oem, ie the passenger tank does not feed the engine but rather just transfers fuel automatically to the drivers side tank that feeds the engine. If it a single tank setup the same dash gauge will still read from the sender in the gasoline tank, the ohm output is the same. Google Leo's Tractors in Wisconsin, that is where I bought our truck. He buys up a bunch of the retired Schwann's trucks and resells them, and converts a bunch of them to gasoline. He can hook you up with everything you need to convert back. Odd duck, but good guy and know's his stuff.
For information on your existing propane setup:
Bi-Phase | $mart Clean Propane - Home
You can download VERY complete manuals on your propane setup, and their tech line is great and they have any part you may need for propane on the truck.