If I had my druthers I'd put a 4-stroke on the Astro Hog. This plane just "sounds" right with a 4-stroke "putting" along. However, I do realize that is not the question you asked. Go with the 75 on it. This plane is a 60 sized plane and I think you'll find yourself lacking with the 55 on it. I'm sure the plane will fly with the 55, but you won't have any "extra" power with that engine on it. The 75 is actually going to be almost overkill for this plane, however this is definitely a case of better to have too much than not enough.

That's my 2¢ worth

Ken