Factor about $600-800 for labor (a day or so) then add up the price of parts. You'd be all said and done for probably like $1400-1700...

Not cheap, but it'll be stout and hold up very well to most anything you can throw at it within reason. That's what it costs. You may be able to skip the hardened input drum and shaft assy but with nitrous maybe I'd try to get it...it's about $300 more. Also the better output shaft will allow you to kickdown from higher RPMs to max out the LS6 cam you're running too.

Again, I'd run a few tests before just to be certain the 3-4 pack is burned up. I'm leaning just a bit more toward to the 3-4 pack now but your milage is pretty low and you're probably making about 300rwhp with that LS6 cam. Stock F-bods make those with the same 4L60E but weigh a bit less. So it's a toss up.

It's either something stupid or toasted.

Do you have access to tools and what not to run diag tests on it?

Have ya posted up anywhere other than on FSC to get other input?