LeBron would be a tight end in football. Do NBA players have the ability to transfer their skills to those of a professional football player, without a doubt. Do they have mentally what it takes to be a professional football player, that's another story.
Football training is a completely different animal and constitutes building mass around your body to withstand the physical beating while maintaining your physical gifts and talents. I'm not so sure most NBA players would want to endure the physicality that is NFL football. It's probably one of the reasons they leaned towards basketball as a child, it's just less physically demanding.
As for your question about tall players, most tall players once they put on the muscle necessary to play end up becoming lineman or TEs or QBs. Richard Seymour is 6'6". I think the Oakland Raiders boast having like 5 offensive linemen that are 6'7" or taller. Football on the defensive side of the ball is about maintaining a low center of gravity in order to maintain position and make tackles. It's just one of many reasons why you don't see giant linebackers or defensive backfield players.
My guess is if Randy Moss, Terrell Owens and Plaxico Burress did nothing but play basketball since they were kids they would have had a pretty decent shot at being NBA players. Could some NBA players become NFL wide receivers, sure, if they didn't mind taking the beating that those guys take.