Questions about converting from 13in to 15in 68 valiant

on stock rims 15X7 4.24BS 205's will fit easily, as will 215's

up front you *might* be able to squeak in 225's if you keep the profile(aspect ratio) low and don't lower the front end considerably. you *may* have to slightly trim the lower front corner of the fender for clearance.

out back, with most stock rear ends you're not going to have any problems until you get up to about 245. height isn't so much of a problem as the quarter panel being your limiting factor-- if you want to keep the tires tucked underneath.

here's a list of widths and more info that can be helpful on determining what widths you're working with.
https://www.forabodiesonly.com/mopar/threads/an-accurate-8-3-4-rear-axle-width-list.365494/
if you've deviated from a stock set up, generally you can use that information to extrapolate the numbers that you would need to select a rim and backspacing. but it sounds like you're not doing anything too exotic, so if you pump the search feature with enough info you should be able to find the info you're looking for.

as an aside, i don't know what kit you're upgrading to, but some aftermarket kits don't play nice with all 15" rims-- reproduction rally wheels and cragars come to mind. so double check before you buy!