Whats
Every car's different. My 3000# door car likes 4600 too.
If you run up nearer the converters stall it does not have as much torque multiplication left to help get you off the line. Lowering the RPM allows more multiplication, especially on a powerglide. It may depend if that puts the engine out of its best torque range.
Like my ol buddy Steve Griner, used to say, " A Powerglide is a lousy transmission with only a 1.76 first gear ratio, until you couple it to an 8" 6000 RPM stall converter, then look out." (That guy knows more about the transmission game than most everybody else, I think. He was a pioneer of the transbrake, the lock up converter, and the 904 3 speeds for sbc Comp Eliminator.)