Your car is what the US guys would know as a Monaco 500. It's bucket seat/console car. Windsor pretty much batch built them usually in mid to late spring. Yours was built around April 22. They predominantly came with a 318-2v, torqueflite, power steering & brakes and an AM radio. A few of them got power windows and maybe a tiny handful were loaded and/or got a 383 or 440. I don't think any of them got a/c, but I'd have to check to be sure.
I think you're pretty safe in looking for the back end of any two door full size Dodge or even Plymouth, if you don't need to replaced the quarter panels. The frame rails and floor itself will be pretty much the same.
"Re-sale" red is a pretty common thing to do with a convertible, so I can see where a PO would have done that sort of half-assed colour changed. I think your car really is that odd shade of green and the "X" in P6X tells us it had a black interior.
There was a pretty sad looking '68 convertible just outside of Windsor for a few years. Did your car happen to come from SW Ontario?