In the late-60's, there was a running QB at the University of Colorado named Bobby Anderson. He was very similar to Tebow size and style-wise. He ended up being drafted by the Denver Broncos and was made a FB, whose main duty was to block for HB Floyd Little. Anderson eventually had to retire early due to injuries, but the 4 years he spent blocking for Little (1970-73) were Little's most productive years.
I think Tebow could be the same type of player as Bobby Anderson. I believe his future in the NFL would be as a blocker/occasional runner/occasional passer.