If you really want a 4L65 to hold up to anything you can throw at it I would
Check out this thread, read all the way through looking at the attention to detail!. Now I didn't go that way and wish I did. Needing the output shaft changed you have to competely dissasemble the transmission anyway. I had a tranny guy do that and the sunshell upgrade for $325. But I bought a low mileage 4L60 for $800. I hope it holds up. If not I will definately send mine to Finish Line! They are building these transmissions for cars in the 600 HP range drag racing. Spend some time at LS1tech as there are a bunch of guys running their transmission.
Now FLT charges a $400 core but many times run a free shipping special and or 10% off for LS1 members. A transmission from them can run you $2-3000 depending what level you go with so it is 2-3 times what I have into mine right now. But if mine doesn't last I will have wasted 1/2 my purchase price (I'll have a core now so I would save $400 of the $800 I spent) And the $325 I paid a local transmission guy for the output shaft would be a loss?
If I wasn't so cash strapped I would just send mine to them anyway!