You will probably be better off to hook the trailer up and see what it will take to level the trailer.

You can change the distance or spacing between the bed rails and trailers gooseneck by raising or lower the gooseneck hitch itself.  Once you have proper clearance, then look at the trailer, if it has lifted the trailer to high in the front. Which puts more weight on the rear axle and less weght on the front axle, then you can decide how much to block the axles to level the trailer.

The distance that is needed between the bottom of the gooseneck and the top of the bed is all determined by your driving and where you drive. My stock trailer has about 9" and I come close on it in the pasture. My LQ has about 6" and that is adequate for where I use it. Same on the RV. My flatbed gooseneck only has about 7" and I have to be really careful on it when hauling hay. Any other time it is fine.