I just finished drag week with a T56 Magnum behind a 392 Hemi in my 31 Plymouth Coupe. This is my first experience with the T56 Magnum. I was holding off on reporting my opinion because I have not as of yet thoroughly evaluated it. But since the topic came up I will let you know my findings so far. With the clutch out idling in neutral and the car stopped, the transmission makes a lot of noise. The noise doesn't bother me, but it makes me wonder if something is wrong. I've heard from others on the web that they are known for being noisy under these circumstances. On the street the transmission is awesome! It shifts nice. The gear spreads are great.
But if you're going to use 6th gear, you better have deep gears or a Torquey engine! I have 4.57 gears in the rear end, 30 inch tires, and at 70 miles per hour in 6th Gear the engine is turning 2200 RPM. This engine has a huge cam, and barely runs at 2000 RPM, so I never used 6th unless I was going 75 mph+.
I also had trouble trying to power shift at 7000, it would hang up. If I lifted off the gas, it shifted reliably, and reasonably quick. But this could be clutch related. My soft lock clutch has an iron disc, which is heavier than an organic or Kevlar disc. Plus counter weight on the levers which can hinder high RPM release. So before I can really make a call on the power shift ability, I need to work on the clutch more.
I hope this info helps.