22 or 26 inch radiator

I believe the 26" radiators started in '73 or so- but not all of them got them. Slants and non-ac 318s still got the 22s.
The best way to tell what it originally came with is to measure the opening in the radiator support.
A 26" rad will physically fit the 22" support, it just may not have the wider attachment points for the 26. And running the 26 on a 22 support is kind of an exercise in futility, since you are blocking off a good chunk of surface area on the bigger radiator- and thus not really gaining anything. Unless, of course, you open up the radiator support to the 26" dimensions.