14" or 15" wheels on '67 Barracuda

I have a 69 Fastback. Disc brake conversion complete and rear swapped too 5x4.5 bolt pattern. Current rally wheels are 14x5.5 with 215/70/14 and a 3 3/4 backspace.
I want to run 17x7 with a 4" backspace. Of course, I have to order online and can't return them when they are mounted... Just not sure if they'll fit. However, the 215 seems quite big for a 5.5" rim....
Any sage advice?