[blykins]

There are modifications that can be done to give between 10-15% more torque on a TKO 600. That would put you somewhere between at 660-690 lb-ft capacity.

If you are truly putting down 650 lb-ft of torque at the wheels (I'd like to see that dyno sheet and engine build sheet), then even with those modifications, you're not adequately covered. Even the Magnum T-56 would not be rated enough to cover that much power.

It all boils down to this: if you're just playing around on the street with street tires, I'd probably leave it the way it is. Tires give up WAY before a lot of stress is placed on a driveline. If you're running a lot on the strip with drag radials or slicks, then you need to swap trannies altogether. You really need a Jerico 4-speed or an automatic at those requirements.

With either one, you need to be running a twin disc clutch setup.