the problem is the cowl...
The problem with the GeeBee is that the cowl is so big, you really need an engine that can spin the prop, say an 11x6 fast enough. My thought is that most of the Jett and Nelson engines that I have seen spin 8's and 9's for speed. I bet your cowl is probably 6+ inches in diameter. My bet is on the Irvine. My .46 rev's like crazy and I have seen a .53 and it is a hoss.
Here is another thought. Why not go with a Rossi or Irvine .60. Spin a monster prop at make some serious thrust.
James