Here's my 360 set up

Stock bore and Piston 360 1980 truck motor
Comp 275DEH
Stock heads, intake, and everything
4160 Holley converted to 4150 vac secondary
Manual shifted 727
2400 stall converter
3.23 sure grip
29x10.5 rear slicks
3200 lbs short box pick up

What would be the cheapest most effective upgrade? I know the gears are holding me back pretty bad and I'm looking for some used at the moment