It won't work. The GP 61(65) is a completely different engine, having a different case, plus different porting & timing from the 61Pro. It is also a better real-world engine than the 61 Pro -- being ~10% lighter & making more power & torque across a broader RPM range than the Pro (which only has an advantage with smaller props at very high revs). A better bet could be to put the 61 Pro carb on a 61 GP.