A few of us here have built our own, and I have built one for my '91. Since you have access to welders and cutters, materials, you are halfway there. All you need to do now is design one, and buy the steel. If you design it carefully, you can avoid using heavy guage and large diameter steel.

A few companies also sell flatbed kits, All-pro Offroad is one.

rustylux

Glad to here you got the steel for free.

Here are some of the early pics. I have since added a covered wagon style frame over the top which will be tied into the exo-cage, and I have added the a rear swing out gate/tire carrier.
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I have plans for battery storage, spare driveshafts, air tanks, etc., in the dead space underneath the flatbed.

More than tread lightly. Leave it like you were never there, nor anyone else.
'90 X-cab 4.88's 33 BFG AT's, rr ARB, Headers, Ignition upgrade, cold air induction.
'91 X-cab 5.29's 315's BFG MT's, rr ARB, custom bumper and flatbed