A little late to post this now but when Gary H. put my '64 together, we sent the '65 3.36 Safe-T-Track diff carrier (came out of a wrecked '65 GTO located at White's Pontiac Salvage in Rolla, MO about 30 years before) to Jim Mitschke of JD Race (monzaz) in Ohio.
jdrace3@gmail.com if he is still active.
I paid $75 for the diff carrier and gearset (after traveling hours to Rolla, the owner changed his mind and initially declined to sell it to me as he thought he might use it in the '65 he was building but after a bit of begging he relented much to my relief).
I don't recall what Jim Mitschke charged to check and cut the cones about 4-5 years ago. I do recall that he was concerned that the gearset was rusted and would likely be noisy. I gave the go ahead to use the gears anyway.
After return of the repaired unit, Gary sent it on to a local guy for assembly into the '64 Axle Tubes and Diff Housing.
I have had no issues with the rear axle since, roughly 1000 miles. If the gears are noisy I don't notice over the sound of the Gardner exhaust.
I was very satisfied with the work done by Jim Mitschke to save the OE (albeit '65) differential. Admittedly, I have yet to spin the rear tires so can't really say how well the cones work. I exercised the cones plenty in '73-'79 and abused the tires in the '64 GTO I owned at that time which was optioned with the STT while my present GTO had the base open diff when it was assembled at Fremont, installing the STT was one of the few deviations from "as built" that I was willing to take.
Just wanted to post a thumbs up for Jim Mitschke and recommend that anybody that has an original cone clutch STT consider having him restore it if the cones are worn.