bc toys is right. If you build over/above the wheels, you'll probably have an extra foot to a foot and a half of width, which you'll appreciate. You could even have extra storage underneath. The 7.5' length isn't that bad. You'll figure it out. Do check out that trailer carefully, though. While a little rust removal and some paint may clean up the chassis, you may need new tires, if they are very old or been sitting in the sun a long time.
Courtney did a great job on her TD. She had the 'can-do' attitude and did it! You 'can-do' too!