Why don't you change your rear end gear ratio. I had a 360 Duster one time and I pulled the 8 1/4 rear end (3.23's) and put in a junk yard 8 1/4 with 2.43's. I cruised that 360 45 miles to work on the highway for a while.
That's fine for an engine with a small cam and an auto. If you don't make any power below 3k and have a 4 speed, you can't just toss in a set of 2.43's unless you like burning clutch.
