Interested to see how this plays out.
I've got a pretty stout turbo build but the trans is always going to hold me back.
This was done back a couple years ago by Pontiac Jeff @enginered performance
Theres another build thread floating around on Bonneville club I believe.
http://montecarloforum.com/forum/sixth-generation-monte-carlos-39/3800-swap-4t65-4t80-31241/
yeah no kiddin....we are getting near 8k invested into the trans of the Turbo regal i built for Heavykevy....and its coming back apart for the non tcc groove equipped GMR Armet 100 input shaft (have fun trying to find one of these in the future) along with the converter getting rebuilt for a 3rd time to machine the TCC groove into the turbine hub.
that way we can keep the 1200hp rating of the IS and retain complete TCC functionality with no loss in strength....believe it or not ...this is the cheap fix....way too risky to modify the unobtainium IS which replacment of would cost ~3500$ each shaft and would require a batch of 20 hahahahaNO. the billet Tq converter is farrrr cheaper at 900$. machining the turbine hub is the better option as that portion of the hub is not under any stress and has plenty of meat to cut the groove.
lifes a ***** when your pushing over 600hp/575ftlbs at the wheels with a POS 4t65e
^^^^^ that being said makes me drool at the thought of a 500 plus hp 4t80 bonneville epic sleeper!!!!

I'm right there with ya. Haven't done the trans stuff yet (have a lot of parts but nothing installed) but if an 80 proves to handle the power I'd look into swapping to that. Also the clearance in the back of the engine sounds good... I'd really like to be able to run a 3.5" exhaust but the pipe simply won't fit between the engine and brake booster.
Are there upgrades available for the 80? Similar to the 65.. stronger IS, chain, clutches, etc, etc...
Here are the 4T65E ratios-
1st - 2.92:1
2nd - 1.56:1
3rd - 1.00:1
4th - 0.70:1
Rev - 2.38:1
Here are the 4T80E ratios
1st - 2.96:1
2nd - 1.63:1
3rd - 1.00:1
4th - 0.68:1
Rev - 2.13:1
There are several overall ratios/final drive ratios also that differ from the 4T65E