I'm thinking the torque converter is holding it back.
Bingo. The stock converter will flash under 3k rpm. You can rev to 900 rpm or 2500 rpm at the line, and floor it at the light, and your 60' time will stay the same (or get worse). Been there, done that.
The biggest help on 60' times is a higher-stall converter.
You already have 373 gears; going to 410's would help a little as well.
Also, make sure traction control is off when you're launching. This can really mess you up at the line!