regarding frame rust, 65 up until early/mid 70's seemed to have very similar frames, but I'd always heard 65-70 were the worst years- sad as this includes all my favorite years (69 hardtop, 67 fastback, 65 fastback).
I had a 69/429 that was rusted thru when my mom bought it in '74, I had a very clean 71 LTD thats rear bumper/rear 3 feet of frame fell off after bumping into Mom's brand new '80 mustang(once she saw her car was OK, she thought it was real funny). and youve seen the mess that was under my 65...
64 and earlier had curved siderails, I dont think these had any rustprone issues, but later had the 'torque box' style and straight side rails. Ive usually noticed in front of drivers side rear wheel usually the worst, not sure why though.
I got some parts from mine from a clean wrecked colorado junkyard car, and aside from heavy rear damage frame looked great- but noticed a slight rust colored line along front bottom of framerail, and pushed a finger right thru it- inside looked like 'graham crackers'- big waferlike sheets of rust you could see thru the various factory holes along the frame...my guess is the car must have came thru the saltbelt enough to collect some salt inside, but undercarriage was typical 'desert dusty' but beautiful condition. be very careful inspecting a prospective car, and be sure to give a good squeeze along the frontsiderails, and rear arches in front of rear wheels- any 'crunchy sounds' means runaway...just be careful- first squeeze on mine a handful fell out! that just may upset the seller :)
also just about all the 'drivers' Ive ever seen around here have all had patches/replacements/reinforcements welded in over the past 40 years- any signs of welding is a good sign its got rot inside. good luck finding a clean one, I know they are out there, but need to find one thats spent its life in low humidity area to be safe. strange thing about these big cars is how strong the body is- Im sure mine was holding the frame up rather than the other way around- car felt tight on the short ride I took in it before disassembling. once body was removed, stepping on front torque box busted frame in half- I need to lose some weight I know, but 200 lbs shouldnt do that :)
heres a link to Russ Owens' frame replacement article- theres a original FoMoCo artical at the bottom: