I will take a swing at it.
Dump bed, if not rusted badly and working and no leaks and installed correctly (as you state) - $1200 (these go about $3500 new now + install (which is not difficult if it is electric over hydraulic, I am guessing that it is.)
Truck - If 4wd works properly, no leaks of fluid anywhere, starts/runs/idles, etc., only cosmetics on interior needed, and little to no rust - $1100.
So, $2300 - assuming that it is a reliable nice looking truck.
If you don't know much about these old squares, just peruse this fourum a bit and you will see that you can count an replacing just about all the wear parts over time, like any vehicle. Some parts are no longer available. Some parts are ridiculously expensive - like a factory dash pad if you can find one. On the other hand, if you are willing to have just a driver/beater you can run these old trucks for quite a while. I personally would look for a 1987 model year to get rid of the carburetor issues (most here will say carburetors are not a problem, but I disagree
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If you really want this truck I would still wait a while. This truck will be a gas hog and a later model fuel injected truck is what most people will want to buy. If you intend to drive it rarely then never mind getting a newer truck.