Yeah no dealership will touch it.

However I think your best route is go with a tuner. I beleive someone on this site has videos on youtube of a tuned 5.0L doing over 110mph. And doing it quite easily at that. I would check out the 5.0L specific fourms, I think he has threads there.

He would be the one to talk to about getting a tune and/or taking off your speed limiter.