17" rim with 5.5" backspace

get a 3/4" - 1" adapter and you'll have 4.5-4.75" BS.

What suspension mod do you have done?

you can fit to mustang any modern wheel (well with proper bolt pattern) and adjust the offset with wheel adapters. The only limitation is the tire width.

Need more info - 17x8" or 17x9"?

5.5bs is normal for a 17x9" for a classic Mustang application, but if it is a 17x9 with that BS I would expect some serious rub that you can't correct with a normal wheel spacer - it would have to be a full adapter with separate wheel studs for the new wheel. 8" width with that BS would be even worse, and I'm pretty sure you're talking about a 9" wheel.

I tried my 17x9, 5.5bs rear wheel on the front of my '70 mustang just to see what it looked like and there was a ton of upper ball joint interference. 17x8, 4.5 or 4.75 is more appropriate.