The bushing should just fit in-between the tabs on the elevator. You will have to file down one or both sides of the stab weldments, possibly in different amounts to get the correct tolerances. Nope, nothing NASA precise about it. You might want to clamp and weight down the stab down to a flat table or door to achieve straight and level. You can pull a fish line or heavy thread from end to end to find straight. then attempt to fit in the elevator as it should while paying attention to where the material should come out. Make this adjustment by finding the best average location across all 4 elevator tabs. This will be a slow process, but you will arrive in the end with a well fitted horizontal tail. During the manufacturing process welding typically deforms materials from stock dimensions.