If you can afford it and you're not in a big hurry I would look into the new 855 transmission offered by Jamie Passion. It should fit without any modifications and is supposed to be plenty strong. Also made in the USA which I think is a plus. Ten years ago when I was putting my 'Cuda together the Keisler Tremec TKO conversion was the only practical overdrive option available. I certainly don't have any regrets for going that way since I would have had to wait a decade just to get on the 855 waiting list. At my age a decade is definitely too long to wait. If I was doing it today the 855 would be my first choice.