I agree with most of what's been stated here so far. I had an old vette that i drove to work 2-3 days out of each week for 8 months out of the year for 6 years, and with a 26" tall rear tire, 3.73 gears, and a 4 speed manual trans (1:1 4th gear...no overdrive), my engine reved at 3,200 RPM at 70 MPH and it was fine. I thought about installing 4.11 gears in the rear, but according to my calculations, if I did that, then the engine would be reving at 3,800 RPM at 70 MPH, and that would be too much for me in a car that was driven 14 miles on the highway 2-3 days per week. It would get to be annoying after a few hundred miles for me. not only because of the constant noise the entire time I was on the highway, but also because as soon as you would let off the gas, it was like hitting the brake pedal. not comfortable at all.
But at 3,200 RPM it was fine. The engine would have a nice growl to it at 3,000- 3,200 RPM, and it sounder real nice, but not over the top. If you go from 3.73 to 3.55 gears, you'll only see a 150 RPM difference at 70 MPH from your present 3.73 gears, so don't even bother. Here's the gear calculator that I use...
I agree that stall speed might have something to do with your issue IF
it's above 3,500 RPM. But if it isn't in the 4,000 RPM range or higher, then that shouldn't have anything to do with how the engine revs when you're cruising with part throttle at any speed since the converter is only going to flash up to the full stall speed under hard acceleration when you're at full throttle or very close to it.
The only way that you're going to see a significant decrease in RPM at a 70 MPH cruise is either switch to an overdrive transmission, install one of those Gear vendors units, or do a rear gear swap to 3.08 gears. The 3.08 gears alone will drop your revs at 70 MPH by 400 RPM. But that ratio will also give you a reduction in 0-60 MPH acceleration as compared to your 3.73 gear ratio that you have now. I think that 3.73's are the best all around ratio for cars that have non-O/D transmissions.