I'll save you the calculator work. These are GM numbers from a 83-84 MCSS. They used a 3.42 behind the THM350 with 26" tires. 215's just like our TR's.
Drive gear(on the output shaft) 6261784 8 teeth
Driven gear 3987922 22 teeth.
There were several different driven gears used. The big difference is what size the sleeve or bullet that the gear sat in was. The number I posted is for the real common Chevy application. The diameter of the sleeve is about 1". Some of the BOP transmsission used the larger 1.5"-1.75" THM400 sized sleeves. Post back if you have the larger sleeve and I'll find a number for you.
The diameter of the bullet looking thing holding the drive gear that goes to the speedometer cable is .875 inches. The ID of the gear inside the transmission on the output shaft is about 1.2 inches and is held on the shaft with a small metal clip. The driven gear is 21 teeth and red i have now and the drive gear is 19 and yellow.
You're going to have to change both of those gears. That drive gear is used on mostly 2 series axle ratios. I couldn't come up with a combo that would let you use that yellow 19 tooth gear. That 22 tooth driven gear will work with your old sleeve.
There was a part number change for 6261784. The new number is 12338033. I dont know what the difference is between those two 8 tooth gears. The 6261783 is truck only so maybe its a typo in the parts catalogs. Maybe it is for a larger diameter output shaft?