If league players were to make it in the NFL, even assuming the hypothetical of sending them back in time to learn the game in high school and college, I reckon most of them would play defence - mostly defensive backs and linebackers, because NRL players generally aren't big enough to be linemen.
Some of the absolute ball running freaks like Uate and Hayne might have made it as running backs, but those guys (running backs) are the best of the best - every black kid in America wants to play running back, and most NFL players probably played running back as kids. It's like how nearly every NRL winger or centre played fullback or five-eighth as a kid.
The NRL players most likely to be suitable for American football (i.e. the most physically explosive ones) would nearly all end up on the defensive side of the ball. The body shapes required for the offensive line and for the receivers just don't suit the NRL, so they would be under-represented in NRL players.