I'd guess 1000 lbs?...more with the fiberglass cap on it,and add another 200 if the doors are on it,and at least 100+ for the tailgate..I'd remove everything that unbolts first!.....and I wouldn't trust the factory roll bar to lift it if I were you--the bar might hold up,but more than likely it'll just tear out of the floor where its bolted in,(where K5's often rot the worst!)--or maybe shear the bolts off,or the sling you lifted it with might slide on the roll bar pipe and make the thing tilt to one side suddenly,and maybe crash to the ground..
I know from scrapping a stripped bare cab off a 73 K10 ,that weighs over 400 lbs alone--add the weight of a 6 foot bed,thats at least another 400,and the roof has reinforcement bars in it too,that could add a few hundred lbs..
The safer way to lift the tub up off the frame without having a loader,would be to jack it up and slide a 4x6 timber under the rear wheel wells and across the front under the center of the rockers ,(use blocks so the rockers dont bear all the weight,especially if they are rusted!)--then use cribbing blocks one side at a time to support the posts a little at a time,till its clear of the frame..then it can be rolled out from under it..
If you have a hoist of some sort,I'd rig up a chain to all 4 corners and pick it up from the "center",but you'll have to adjust for one end weighing more than the other,to be able to lift it straight up,so the hoist wont actually be hooked "dead center"..its more risky lifting it that way,any weak metal could be tweaked and bent before it lifts up..