If you smell LP and you can get it to run at all, turn off the LP at the tank while it's running. If it eventually smooths out and runs fine for a short while before running out of fuel, you know you're getting too much fuel from the regulator. If you've got a Model J or K regulator, I'd replace it with a Cobra.

If you're planning on keeping the truck I'd replace the distributor with an electronic one. Maybe that's just me though. I adjusted so many C500 points back in the day that I still sometimes dream about doing the job. The complete distributor went for $100-150 when I last looked. (years back)