I would also do the 4L80E and be done. Ive been driving on the 80E swap for 5 or 6 years now and no regrets.
I did everything myself except for the ujoints which my buddy did. i payed for his lunch that day so it all worked out.

 

I still say go 4l80 for your build but ill tell you tho the 6l80s are pretty stout. Mine is completely stock and I have ran several setups through it without ANY problems. I have done a 5.3/nitrous setup, 5.3 turbo setup on 7lbs, a 6.2 turbo setup on 10lbs and now I have my 427 setup that currently is on a 150 shot without any problems yet. Next will be my 7.0 with twins here shortly so will see just how far I can take it. So far from what I have seen is keep trans temp low and GOOD trans tuning. That to me is the biggest factor.