Set the engine in to where it looks right and DON'T worry about the bubble in the level being exactly between the marks. If the carb had to be perfectly level then vehicles couldn't run uphill, downhill, or on a side slant. No one in San Francisco could have a car. One should have the centerline of the crankshaft close to the centerline of the pinion to prevent driveshaft vibration and wear. There is a lot of posts on this subject but basically you can get a $15 angle finder from Home Depot and set the pinion yoke so it's a degree or two down from the reading you get from the transmission output shaft. This allows the needle bearing in the U-joint cups to roll and prevent flat spots. Take your time, measure carefully (twice), and your project will turn out fine. Good luck

Evan