does anyone have or had a SD prop on a LT??

I don't think that'd work out to well for you. The sd's are geared down, to swing the bigger wheel. I don't think there's any way possible that you have enough motor to swing a prop that big.

Yeah, they have a reduction in em. Thats why the 18 lt swings a 9" prop, I believe, and the sd swings the 12".

This taken from the gd site:

The GO-DEVIL® Surface Drive Engine features the toughest frame in the industry, consisting of the perfect mix of steel components that will dominate all environments. Inside the frame lies the drive train. Designed with the sharpest of pencils, the drive train was engineered, hand built, and rigorously tested by our resident mechanical engineers and skilled shop hands. At the heart of the drive train, high performance gears connected through precision belts mechanically reduce the engine's revolutions, thereby generating more torque to turn a larger mud slinging propeller. Because of the overwhelming success with the components used in the drive train of the modern GO-DEVIL® Long Tail Engines, the new GO-DEVIL® Surface Drive was crafted with many of the same features in the shaft housing including the upper bearing assembly, the pressure lubricator, and the ever-dependable seal housing and lower bronze bushing, now with a spiral groove for positive pressure lubrication. Additionally, the new GO-DEVIL® Surface Drive utilizes a constant velocity joint that eliminates undesirable engine angle. At the end of the drive train rests the latest in propeller design and innovation. This new propeller is designed to cut and "sling" through the thickest mud. The new GO-DEVIL® Surface Drive will be available in a variety of Honda, Vanguard and Kohler engines and will carry a two-year manufacturer's warranty.

That would be correct. No reduction on the LT.