RE: Right engine for Extra 300 (.60)
Plugin,
I'm just finishing a review of the 75 AX for RCU and I can tell you that this engine is a real "stump puller". Putting the 75 in either the GP or Goldberg Extra 300 will give you more than enough power, since you say you don't want to 3D. I think it would be a great combo for either plane.
Ken