I really don't understand people having problems with say 3.55's on the highway, especially in a 400. 3000rpm at cruising speed is average to low for most modern (4 cyl) cars, the difference is they have decent sound insulation and/or quiet exhausts. If 3000rpm on the highway bugs you, try some quieter mufflers. A lot cheaper than a rear end swap and no loss of acceleration.
A good friend of mine has a 455 powered lemans with 3.31 gears and a th350, 80mph cruising is not a problem and he is going over 3000rpm. He's made the 8 hour trip to Norwalk 3 times. 3.31's would be even less a big deal in a 400, but 3.55's would give around the same effective piston speed etc.
Fwiw, my '78 t/a has 3.23's and a 400/4 speed, I get 20mpg highway mileage and if I were not into road racing vs. drag racing, I wouldn't hesitate to go with 3.73's.
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