[dsraven]

I would say figure out what it is you want from the truck. if you are going to keep the solid axle up front then it makes no sense to upgrade the rear to a 4 link. the reason for a 4 link is to help the rear axle "hook up", to be able to stop wheel hop under hard acceleration, to be able to adjust the weight distribution when under hard accelleration, to be able to adjust pinion angle, to be able to easily adjust the ride height without messing with the pinion angle etc-like bagged vehicles use, or to be able to quickly dial in a race car suspension, again, for traction. note that almost all of these are for traction control. if you plan to swap out the old inliner for some high horsepower unit, or plan to install an IFS up front so the ride height will be lowered, then possibly a 4 link would be nice to have. a 4 link also comes with a few drawbacks, one of which is the removal of free space back there for exhaust, fuel tanks, etc. there is nothing wrong with a set of leaf springs. possibly a few less leaves than the stock pack would help with the ride quality. since you will have the bed off you will see that other than requiring a lift or some help to removeit, that job isn't all that tough. a new bed would hopefully have new hardware fasteners with antiseize so getting the old rusty fasteners off won't be an issue. possibly clean up the frame etc under the bed and apply some fresh paint, consider placing a fuel tank outside the cab, upgrade the brakes to have a dual master cylinder for safety, etc. things that won't take away from the vintage look of the old truck with the stock inliner with the cool supercharger.
if it were me, with the box off, I would:
(keeping in mind that anything removed from the truck would be properly boxed up and labelled so the truck could be put back to stock in the future if so desired by yourself or a future owner. taking pics of the before and after is also a consideration)
-strip the frame down to allow buffing and fresh paint
-scrape, wire wheel, sand or whatever needed and apply a nice coat of frame paint
-upgrade to a newer leaf spring set up with new hangers etc for a softer ride with rubber bushed springs. no more greasing and squeaking and a better ride. bear in mind the pinion angle when doing this step
-some new shocks for the same reason as above
-install a rear stabilizer bar
-ensure the rear brakes and park brakes are up to snuff. including the brake lines and hoses
-ensure the rear wiring is all in good shape, tied up in loom and secured well. possibly install sealed electrical plugs for ease of removing the box in the future and to keep everything sealed up
-consider a ower brake unit frame mounted. personally i would look at the hydroboost since it isn't affected by engine vacuum or boost

after that the front end would be my next consideration. it all depends on what your plan for the truck is though. if you want to keep it all along the idea of keeping the stock truck with the cool old inliner then that will hamper what you can do with the front suspension. a front disc brake option would be high on my list. power steering would run a close second to that since I drive mostly city miles

anyway, if you want to do a 4 link set up i suggest to look into some of the online 4 link calculators so you have an educated idea of how the systems work. if not done correctly it can make the ride worse and really affect the traction of the "light rear end" truck. 4 links also require some sort of spring so the frame may need to be beefed up to allow that new configuration to work without problems