Bob Hanlon from St. Peters, PA was at the London Cobra Show this year. He is listed on Tremec's website as an authorized distributor.
The 5&6 Speed Specialist | Hanlon Motorsports
Has anyone had him do any transmission work?
Talking to him, he said he changes TKO 600 5th gear ratios all the time, so he always has a supply of both ratios in used parts. If you are willing to trade parts he said he can save you about $300. New parts is ~$600 and he can swap you parts for about $300. I believe he said around $300 labor to go through a TKO 500 or 600.
He can also do all types of upgrades for any intended use from street to racing. He seemed to know these transmissions inside and out. I would like to hear from anyone who has experience with him.