I agree - they are awful stiff. We had a '59 Imperial in the shop that had a 392 hemi in it and I was amazed at how strong the frame was. We custom made a disc brake conversion for it using a Dodge truck rotor (same bolt pattern as Imperial), custom brackets for the calipers and a custom spacer...
Long time lurker, just took delivery of a 1963 Imperial Crown, all original and runs nice. Need to change belts/hoses and check brakes before any cruising.