@JohnnyA, I am no means an expert on transmissions. For me to go to a 4l80 was way more work than to rebuilt my 60/65 to handle the power of my build (~500hp/500tq). From my understanding, both transmissions have flaws that need to be addressed in a rebuild. As was pointed out, the 60/65 is a lighter duty transmission from the factory. There are after market parts to make the transmission handle 800+ hp. There are even companies who will build one for you, like
Performabuilt. For my project, having to buy the custom input shaft for my NV149 transfer case, custom drive shaft, repinning the wiring harness, fabricating the new transmission cross member, was more work that I wanted to deal with and in my opinion was not the right answer for me. The 60/65 will never be as strong as an 80, but ultimately, I did not think i needed the 80 just like the 6.0 engine will never produce low rpm torque as good as a diesel but i did not want to do a diesel swap in this rig either.