The 455 Olds is a great motor. It may not be historically correct in your 68 GTO but it was available in the 68 and 69 Hurst Olds when GM put 400ci as the limit on cubic inches for the mid size cars. Also there was a 1hp per 100 pound rule as well. These were GM rules and Olds was the first to break it by sending the cars through Hurst. Sort of like the Royal Bobcats, Motion and Yenko cars.
I would drive it and have fun with it. I knew a guy 20 years ago that had a 68 Barracuda with a Chevy 350 in it. That was a conversation car.
Anyhow for tons of info go to
www.442.com and see the FAQ's pages
www.mondellotwister.com
www.realoldspower.com
John