If you search on cab mounts or frame stiffening I think you'll find a lively discussion that was had a while back regarding frame flex. The older frames flex quite a bit especially in torsion and so the spring mounts are required to keep the bolts from ripping though the sheet metal and or wrinkling the cab. If you stiffen the frame while you've got it apart (and aren't worried about keeping it all original) then with adequate stiffening you could skip the springs and go to a solid mount.

'69 1/2T stepside, Sea Foam Green, 250 L6 w/ 3 on the tree, not an option on it, in the family since new.