ya, theyre unibody like old mustangs. id just cut out everything thats rusted and replace. the front torque boxes and rear frame rails are where ou really need to look. alot of people add rails to tie in the rear frame rails to the front torque boxes on these old fords

putting a whole new frame in the car would be much much much more work

just do it the right way. if you could build an entierly custom body over frame and suspension then im sure fixing this the right way shouldnt be a big deal
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1989 Grand Marquis GS