In that case I'd put the web address at the bottom rather than the "horsepower.....". Getting your customers to wear advertising for you on a giveaway basis shouldn't be too hard, but might not work as well for a for sale piece. It's not like "bragging" that the wearer has an Isky Cam or some such well known product or favorite celebrity driver. Perhaps something that appears more generic, but still gets the message across. Say "Cool Custom" in the same font and size at the top with a smaller "Graphics" below that (still above the oval). Then the web contact across the bottom. That way the potential buyer would feel like their primary expression is that they admire custom cars, but those who take enough interest to look closer will be inspired to contact you for a commission project.

If you aren't already thinking about it an ideal market, especially for the work shirt design I'd guess, would be the "Billet Proof" circuit of shows, and "Viva Las Vegas", or any other rockabilly deal. Both those shows have websites.