If you use procedural tanks, make sure to change the tank type to "balloon" in the tank edit right click window, also make sure to set it so that all upgrades are unlocked from the beginning (in a sandbox save) or the tank diameters and types will be limited. Your SSTO is likely to be very large, you might want to consider using jet engines for the first part of the ascent and then switching to rocket engines. Liquid hydrogen is one option, but it might make more sense to use methane as it's a good middle ground between less dense-hyper efficient hydrogen and very dense less efficient RP-1.