Need a little help deciding on the right transmission....

I'm working on a '67 Mustang GT fastback for a customer. It is a street car and might see the track once just to see what it will run. Originally was a 390 4-speed, then switched to a 428 cobra jet with a z-spec T5, and now we just installed a Keith Craft 464 (still a FE motor). We need to upgrade from the T5. The motor makes around 580HP and 600 ft lbs of torque. At first I looked at the G-force T5 and the Astro A5 T5 but torque capacity is only 500. Kieth Craft uses the Tremec TKO 600 behind these engines but they were quick to tell me they shift like crap. I have a Tremec 3550 TKO in my car and yep, it shifts like crap. We would like to stay T5 based if possible because it's a bolt in. There's a bell housing adapter and a custom crossmember in the car to make it fit and we would like to get by without having to re-engineer the car. I noticed the richmond 5-speed is priced around four grand. Automatic is out of the question and it needs to be a fairly quiet and roadworthy transmission....Which direction should I go?

you could always get a g-force prepped t-5

G-force torque capacity is only 500 ft lbs.....engine is over 600. I've been reading where the mustang guys are running in the 9's with them so the torque capacity rating may be a little conservative.