I do not think Anyone has an answer as to how much Nitro will it take to hurt the bottom end..Not trying to sound smart but you will know when its much too late.
What many people don't understand about Nitro ( except people who do) is the Burn rate on this stuff.. With Gas/Alcohol you have an explosion basically like a hammer pushing the piston down...The Nitro will keep exploding ( expanding) the full length of the stroke and will try to push the pistons thru the bottom of the oil pan..thats where your rotation of the crank comes in..so guess what takes the brunt of this Massive force...Yep .....rods,pistons,crank etc...
This is one of the arguments the NHRA Funny car guys are having for wanting to run a Bigger Percent % of Nitro..they claim they can lower their static compression therefore lessoning the amount of hit ( hammer effect) at the top of the stroke..therefore saving engine parts...
this touches your question about 40 lbs of boost etc...OF course i cannot compare my stuff to their stuff ( NHRA)..i can't speak for you or anyone else.
Myself Being a 3 time owner of Alcohol Blown Injected cars i would recommend leaving the Nitro ALONE...If you got a Big ( Good) enough Roots type blower to pump that boost over the 30/35lb mark..your gonna have enough challanges keeping it together....Then again if ya JUST GOTTA know 1st hand of NITRO..there is no better experience than the school of hard Knocks ( no pun intended)..lol
IMO.
Brian