If you lower your gear ratio at the rear end your gonna lose bottom end acceleration. The car will become a real dog off the line in other words. If you still have the stock 2.72 gear in the rear end, a gear change isn't really going to help you out. I'm not even sure you can get a lower ratio for that Dana 44 IRS unit. There was a 2.72 and 2.87 gear offered in '81 and '82. '80 vettes all had 3.07's.
So the only way your going to get the results you want is to bite the bullet and spring for a overdrive kit. Expensive yes, but the final result will be worth it especially if you are going to drive the car a lot like I do with mine.
Speaking of your license plate, good old "dads2vette" from Hamburg, NY has a similar plate. He stopped by the shop for new u joints in the halfshafts. Here's a pic of his car w/ plate "GRTWHT81" Dave is the one on the right. The other guy is the riff raft that stops by every so often. lol.
______________
GEN III 5.7L "LS6" Engine swap
TKO500 5 spd.
3.54 Dana 44