Speedometer gear Question

I tried posting this in the other trans form but it kept blocking me so here i am...

I have a 727 in the shop being rebuilt for my 68 barracuda that originally came with the 904. it now has the 727 (year and source unknown to me) with a 3.55 sure grip rear end. the speedometer needle kinda dances up and down when i drive so i think it could be a bad calble, bad speedo gauge or maybe a bad gear??? the cable seems ok so the speedo gear is the easy fix right now so i'd like to try it out. Does this seem like a probable cause for the "dancing" of the speedometer needle?

the trans has a black 32 tooth gear and i have some 15 x 7 cop car wheels that ar going to go on soon. Does anyone know if this is the correct speedo gear for mr setup?

I know tire size matters but haven't got that far. i know i will be putting in the biggest tire i can run with SS springs if that helps.

Thanks for the help,

Robert