I agree with Anthony about a later chassis. An earlier chassis from a straight axle as a retro fit would be a lot more work to wind up with a lot older technology and problems making the alterations to get a 65 & later cab to bolt up you must relocate outrigger mount brackets. And the steering is a "Monkey Goat Rope" to work out "after the fact".
My 66 SWB is on a 79 F-150 SWB Trailer Special chassis. It's really a improvement in my case because it's equipped with 14" x 1.25" rotors & double piston calipers, like an F-250 has but keeps 5 on 5.5" wheel lug pattern of F-100 & F-150 family trucks. I assert use of a later 73-79 style chassis is worth all the work to git 'er Dun because of all the great radical improvements in steering, stopping & handling & with rearaxle being same width as the front wheels, not -2 inches like 61 thru 72 are. Also the 73-9's often came with the more desirable 31 spline axles in their rear 9" differentials and WAY BIGGER rear brakes too.
There is a slight difference in Wheel Base lengths. I'm not certain what a WB is of 73-9 LWB is compared to that of a 65-72 LWB. I know from doing mine there is +2" in SWB's. The difference appears in CA area, or Cab to Axle Dimension. Everything else remains the same dimensionally speaking. Also Fuel tanks are in rear between frame rails by the spare tire where they belong, and you get free space behind your seat for your tunes, or guns or lunch or luggage or what-ever . . . . See why I went for 73-9, upgrade swap?
A little "fudging" and a new set of holes and the bed will bolt down fine.
FBp 