Jack up cab a little, take out all mounting hardware, spacers, rubber, springs if any and keep track of what goes where and you will see what has failed/disintegrated, missing, collapsed due to rust, etc. Then fix it, then patch. It's not obvious but obviously something is wrong, so that part is obvious.
Martin and Dave have already given the advice and info. I agree with all of it. As said, you said frame off (which is really cab off), so therefore you will be removing cab and will be replacing all cab mounts.
Usually a "frame off" is more of a complete restoration not with junk yard parts?
Anyway the best approach to this crooked deal is to take a whole bunch of measurements BEFORE taking any thing apart.( Linear measurements, plumb, level and square measurements). Floor to chassis in several places. Chassis to cab in several places. Axle to frame/chassis in several places, front and rear. Write it all down with a sketch and take many pics of parts coming off and their hardware. You WILL forget if you don't. These measurements may identify what is crooked, sagging, broken, assembled wrong by previous owner and dog tracking.
ALWAYS state what truck we are talking about...year make model!!!!!!!!!!!!!!!!!!!
