What rim size can I run?

Im looking to swap out my stock 14 inch rims on my 73 Valiant. I would like to go with 15 inch rims and wanted to see if anyone knows what size rim I can run. I would like to run steel rims still to keep the dog dish look. Ive always hated how the rear is narrower then the front wheels. It looks like there is an 1 1/4 from the outside of the tire to wheel well in the front and 3 1/4 in the rear. The front is fine but it would be nice to go out 2 inches in the rear. I am currently running 215/70R14 Tires.

What is the stock width of 14 in steel rims?
What is the stock width of 15 in steel rims?

Thanks
my brother with the bbp set up runs 225 60 15x6 or a 215 65 15 will also work with 4.5 back space and 255 60 15 15x7 4.5 back space

the reason the front has alot of back space is because BBP disc widen the track up front so we had more back space to tuck them in. This is with BBP pattern