Even with the power absorbing loss suffered by the GTO vs. the Vette or F-Body, you'll still need over 330 rwhp on the dyno to be putting out 400 crank horses on an M6.
My car did 335 on a dyno with just a filter, exhaust, headers and a more agressive pump gas tune and later, the addition of a moderate cam and fairly mild heads got her up over 375 rwhp, for over 450 crank horses, give or take.
Aside from adding horses, you can shave weight and increase traction as ways to get your speed up. I think I saw somewhere that a very high level rule of thumb is that every 15 pounds is about 1 hp and that every 7hp is a tenth. Ergo, every 100 pounds you cut is like adding 5-7hp, plus you increase your traction, especially if you are dropping weight off the front. Lighter wheels, skid plate removal, fiberglass hood, and you've gained horsepower, increased mileage and boosted your traction. Put some grippy 275s out back and you've effectively increased your ability to get down teh 1320 even more.