Liberty in the US would be the main place. Either buy already done from them, or if you had a TKO you could send your gears to them for the modifications. I did this with my T56.

http://libertysgears.com/our-products/tremec/

One good benefit of doing this as opposed to a full dog box...is you have the option of leaving 1st or 5th as synchro.

For a regularly driven vehicle, leaving 1st as synchro is a good thing, even more so one that might see a lot of traffic.

And if your racing never sees you shift hard into 5th, again, leaving this synchro can make driving the car much nicer.

Not a bad explanation here lol


Or a comparison with T5 and TKO.