Bed bolts are a crapshoot. I've had a couple or 3 beds off and on various gen1/genII Rangers (and several in the wrecking yard picking up replacement bolts and clips). I've seen bolts from older Rangers come off in reusable shape and newer ones rusted to broken/useless.
The advice above is good. PB Blast or other penetrating fluid (from underneath, yeah?) and let soak in if possible. Usually they'll be stiff to break loose, go easy for a bit then start to creak when the rusted ends hit the clip threads. When this happens, work them back and forth a bit rather than strongarming them til they break (judicious use of an impact makes this much easier, just avoid going ape and busting them off - though sometimes this can't be helped.
If you do break them off, you can usually get them free once the bed is off, with vicegrips/sidegrinder and some cussing. Avoid making enemies with the driver's side bolt right behing the cab - the gas tank blocks you from reaching this one underneath.
When harvesting 'new' bolts and clips at the boneyard, remember bolts from a wide range of years will fit (at least GenI/II/III). I usually take the bolts that will come easily (likely in best condition). If I can get them all, I lift up the bed sideways and use a 2x6 or something to support (be damn careful if you do this - DO NOT crawl in under the raised bed!) and get the clips from the exposed side, then raise other side (release fuel tube) and go after the others. Remember the two bolts from right behind the cab are longer than the others.
If possible get ahold of a 12mm die and chase the threads of bolts you are reusing.
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