I'd forget bedlining the frame, if your not perfect with application, and make sure it is 100% rust free, and 100% coverage and 100% adhesion, you'll end up losing the frame before you even realize it's even rusted. Bed liner has its place, and doing things like floorboards and rockers are ok, but a frame I would rather see any damage and spot treat as needed.
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1985 GMC K1500 long bed, 6.5 TD W/ NV4500, NP208
1991 GMC K2500 6.2L NA W/ 4L80E