I read the first 20 pages of this thread, then couldn't wait any longer to go to the end. It is apples and oranges, but on the other hand they are 2 contact sports, with tackling and ball-running, that both evolved from rugby union.
Some random observations:
NFL players are bigger, stronger, and faster than NRL players.
NFL is more anaerobic, RL is more aerobic. But that doesn't mean an NFL player couldn't last in an RL game, given the proper training. The running backs, wide receivers, defensive backs and linebackers are superior athletes who could adjust to another sport's fitness demands easily. Many of the biggest linemen couldn't run a long time at their current weight, it's true. But some linemen are super athletes too - defensive end Julius Peppers, 6'7", played basketball at the U of North Carolina, lots of running in that sport.
Even the best NFL runners couldn't switch to RL easily though, because they didn't grow up learning the ball skills and tactics of league. They'd be athletic enough and tough enough to do all the running, passing and tackling. But I imagine there would be too much to learn for even a super NFL athlete to play at the NRL level. Not that it would ever happen, because there is too much money in the NFL for a top player to ever want to switch to RL.
Which brings up Manfred Moore. I followed the NFL closely in the 70s, and I never heard of him except on this forum. So he wasn't a top NFL player. And he was in his 20s when he tried RL, I believe. But if you could take a Culpepper, Randy Moss, Barry Sanders, Junior Seau, or someone like that, move them to Aus at age 15, say, and start training them for RL, you'd end up with an NRL superstar.
