need help with speedo calibration

[INSIDIOUS '86]

so i just swapped my trans from a 700r4 to a stout th350 and camaro gauges. the long short is that the shop must have got lazy and threw in a green driven gear, and a green drive gear and the driven gears teeth got smashed. luckily there was no damage to the drive gear. so right now a have a brown driven gear in there just to make sure things work, as it stands i register about 20mph on the speedo at aprox 35-40 actual.

my setup has 2:73 gears out back and 275 50 16 tires that are about 26.50 tall
anybody know which gears i need to change to and what?