I've heard talk of the Passon being priced around $4200.00. Still pricey but considering the prices of the other transmissions it might be well worth it.
According to the various articles I've read, they are claiming a solid 650 ft/lbs rating, and they consider it to be very conservative. If you look at the pictures of the Passon 5 speed you can see that the gears are much larger than the A833 and the bearing surfaces are larger as well. So I think the 650 rating is probably accurate.
The best part of the Passon unit is that it requires NO hacking of the floor. If you've followed any of the build threads regarding the T-56 you'll notice that it requires fairly significant surgery. Now for someone like me with a slant six car with little value, cutting the floor isn't a big deal, but original 340 Cars or 383 cars it would be a crime to hack the floor.
As far as the TKO 600, I don't think it will actually hold up to 600 ft/lbs AND sticky drag radials that actually hook. I've read numerous stories of exploded TKO 500 and TKO 600.
Regards,
Joe Dokes