I don't think an M100 engine would fit in a GT40...although it can't be a whole lot different than a BBF 427 engine was, and they fit....
Out of the box, the M100 had 4 bolt mains, a forged crank, and essentially a bulletproof bottom end. Karl the German looney tuner in PA has built them to ferocious power with Vortech blowers, EFI, etc. I think the length of the cam chain may limit rpms, but the performance he has gotten from these engines in Mercedes 600 limos is amazing. They will smoke the tires. One of his cars was fitted with a 5 speed ZF manual, since it would cook the MB automatic box's clutches. His website is
www.MBGrand600.com or something like that.
They are not light engines, though. The later MB V8s with aluminum block, EFI, and AMG parts available would be a better choice. None of this stuff is cheap. The V8 motor was always the cheapest part of the GT40 anyway. For bang/buck, plus low weight, an aluminum SBF is the best, if you ask me.