If this is what your front brake setup looks like than it's a 5 x 4" bolt pattern...
http://i1127.photobucket.com/albums/l637/muumuu101/1968 Dodge Dart GT/101_0016_zpsbd0913d8.jpg
If the calipers look different, you may have the newer brake calipers off of a 73+ car and that would be 5 x 4.5" bolt pattern.
As far as wheels go... The only two companies I know of that sell 17" wheels in 5 x 4" bolt pattern (that actually look decent) are Coy and Motech Performance...
Coy C-5:
http://www.coyswheel.com/coys.html
Motech Performance's wheel options:
http://motechperformance.com/store/
Overall, both these wheels are fairly affordable and come in 17" or 18" wheels; however, their offsetting isn't too ideal for A-bodies. Since you want a 45 series tire, for the front you should be able to fit a 215/45/17. You may get lucky with a 225/45/17 with a little work. For the rears, I think it's comfortable to say you may be able to fit a 245/45/17 without modifications.
If you have a 5 x 4.5" bolt pattern you'll have a wide variety of wheel options to choose from. Just make sure you measure and choose the right backspacing for your application.
Just in case, I like to bring this up, the added weight toward the outside of the wheel (also known as "mass moments of inertia") may add a little bit more load to the rear end. So, if you have a 7 1/4, I'd be cautious on abusing it. Some have good luck without exploding them, others don't.
