Biggest tire I can run on a 1964 plymouth valiant

So, you've got what, about 6" to the springs from the WMS and about 5-1/8" from the WMS to the quarter lip? So let's just call it 11" of space to deal with since your tape measure isn't squared up. Section width on a 245/60/15 is 9.8", so you basically need to be smack in the middle, maybe a smidge closer to the springs.

Take your 6", subtract a 1/2" of clearance, you're at 5.5". A 7" rim is actually 8" outside to outside, so .9" from outside lip to the edge of the section width. So you're looking for a 15x7" with no less than 4.5" of backspace. That also give you 3.5" of front space, plus your .9" of tire, so 4.4", and you've got .6" to the quarter plus a bit.

Hopefully you measured the "tight" side or both sides are the same, if you did then a 245/60/15 on a 15x7" with 4.5" of backspace should be the ticket. Unfortunately, that's not a stock size, and a 15x7 with 4.25" of backspace will mean you'll be too tight to the quarters. It'll clear, but, it'll rub with any body roll- so, corners, speed bumps, somebody in the back seat or some extra junk in the trunk and it's gonna rub. Also, a 255 on that same 15x7 with 4.5" of backspace would probably work if everything is dead nuts accurate. The wider 10.2" section will probably end up being more like 10" on the 7" rim (255 is measured on a 7.5" wide rim), so you'd be right at a 1/2" clearance everywhere. Basically the minimum to the quarter if you don't want to rub, a little generous to the springs but not by much.

If you were running drums, a 15x7 with a 4.25" backspace would likely be the ticket. The disks add some width to the rear track, the rotors are thicker where they sit on the axle flanges than the drums are. It's usually just under a 1/4" per side that gets added with the rear disks.