T-10 Transmission in a '68?

I am almost done refurbishing my 1968 chevelle 300-deluxe. The guy I bought it from sloppily converted it from an automatic to a 4-speed. I now have a Borg Warner T-10 transmission conflicting with my original speedometer. The two dont match up and are not calibrated. I dont know how to fix it cause the speed difference is not consistent. The speedometer is very erratic and I've replaced two broken cables within a week. Anyone have any ideas/solutions???

Typically broken cables are due to a defective speedo unit. Some rotating parts may be binding.
Depending on their design, many speedos can be rebuilt and recalibrated.
I have fixed binding speedos on my own, its not very difficult, so long as the housing permits disassembly.
IMO I suggest you get the speedo working properly and consistently first. Once that is done, if it's inaccurate one can get speedo drive adapters to compensate for gearing changes.

Did you replace the cable, or the entire cable/housing assembly?

While you're at it, undo the cable at the tranny tail shaft and see how the drive gear looks, you'll know right away if that's what the problem is.