How much torque can a 3.55 ring and pinion take?

Its probably the rating of the transmission that they are talking about. No dealer will turn it past that rating.
You have to go to the aftermarket to get it turned up past the trans rating. I would not let the driveline rating stop me from turning a truck up.

 

Never seen ring gear problem, trans yes.my mechanic built twin turbo e model. Put 1402 to roller before input shaft broke.

 

or possibly thats as high as that cm will go without major internal modifications

 

The input shift is usually the one to break.

 

Liability, if he turns it up over the rating and the trans breaks, who's responsible for the damage? Say for example, you just put that trans in the truck, and it has a warranty, you have it turned up over the rating and it breaks, now the trans warranty people find out it's turned up and void the warranty, who get's blamed, you for telling the mechanic to turn it up or the mechanic who turned it up?

 

The guy who owns the truck. It's really that simple.

 

You're fine, just don't drive like a lunatic and your entire driveline will handle a lot of power.