As I have already made mention, the '64-66 F250's are very unusual trucks.. If the truck you have is in reality a factory 4x, in lieu of a backyard conversion, the chassis is more similar to a 1 ton or bigger truck. The springs will not have rear shackles, they will have progressive roller blocks.
The wheel base will be shorter than a regular F250, therefore the bed will either be a step side or if it is a fleet-side, it will be the '57-60 style, being just barely 8' long. The '57-60 bed looks a little odd on the truck because the lines do not line up with the cab.
Ford came out with the Twin-I-Beam front end in '65, however they did not use it on the F250 4x's.. Since the front 4'x axle is a solid axle, Ford used the earlier '63-64 style of cab.. You should note that the cab has interior "running board" steps, like the '57-64's.. The '65-66 trucks do not have these inclosed steps, because they interfere with the trailing arms for the Twin-I-Beams.
If your truck has any rust-out in the cab, you will have to use patch panels from a '63-64 Ford.
You have indicated that the truck has rust-out below the tail-light, this would imply to me that the truck has the '57-60 fleet-side bed,, large round tail-lights..
Your stuck engine could be a problem.. I would suggest that you pore a liberal amount of Marvel Mystery Oil down the carb.. Let it sit for several days, then remove the plugs, and try to turn the engine over... Let it sit some more after you squirt MMO into any cylinder that oil did not run out of..
The 352's have not been made since the mid 60's, the more common FE engine is the 390, which was used my Ford up through the late '70's in most of the Ford trucks..
Only the FE series of engines will fit the bell housing in the truck you have.
The trans in your truck with be the New Process T-18 4 spd.. A very good transmission. The front axle will be a Dana 44, the rear axle should also be a Dana 44, however, I have seen a few Dana 60's in the F250-350's
If your truck is in fact a Ford factory F250 it is quite rare, one source I have read claimed that only 1,500 of these truck were built.
I have attached a pix of my '66 F250. the pix shows the truck in the "as found condition".. You will note the early style fleet-side bed. The truck also has front sheet metal from a couple of trucks. All of these make do repairs have been discarded in favor of correct '66 Ford parts.