bullitt wheels on a 70'????

This is a pretty common swap (late model Bullitt wheels onto vintage Mustangs), so its been pretty well researched. Typically you would want 4.5" backspacing for a 17x8 wheel, but as you've discovered, late model wheels come with a 5.72" backspacing. To solve this, many of the Mustang parts suppliers have started carrying the "correct" spacers (actually, they're adapters) that are 1" thick. This brings the effective backspacing to 4.72", which should be fine in most cases (especially for a '70). You can get the correct spacers from places like NPD or similar. You should also check out www.karkraft.com . They've got the spacers and wheels too.

Hope this helps...