The fumes are unburned hydrocarbons. The easiset solution is to make sure your engine is in good condition and tuned up properly. Bad rings, valve stem seals, etc will leak oil into the cylinder where it will burn and smell.
Tuning it to run rich will give you more power (up to a point), but also result in unburned fuel and fumes.
Mis-adjusted timing will also result in unburned fuel and contribute to the smell.
If you have not given the truck a good tune-up and adjustment, I would recommend starting there. Some kind of a fuel injection system will help if tuned close to stochiometric (14.7:1), but the overall condition of the engine will have a larger impact.
I would disagree with Otto on the catalytic converter. Trying to run one with your old carb is highly unlikely to work long term. Putting one on with a fuel injection system that is more consistent in its fueling should be fine. Modern cats don't cause the performance hit that they did 40+ years ago.
I personally like the aftermarket fuel injection systems (and you will get mixed opinions around here), but they are not a panacea for all that ails your drivetrain.
I will warn you that propane conversions are a strict No around here. You just don't want to go there.
As far as a body swap, that is a lot of effort but you gain modern drivetrain and suspension. Depending on your long term goals and what you are willing to invest, it can give you a really nice rig. But again, it is a lot of time and money if your real goal is to just reduce fumes.
