Draining the tank at least 3/4 is recommended....I would drain it down (if it were my tank...and I didn't want to take the fish out....which I hate doing...drain it to give the fish just enough water to cover them.....they will be fine...) plastic shims wherever you get them just don't compress....so you will never have enough weight to crush them....wood on the other hand may. Shim it up and fill'r up. If you need to adjust, repeat process....I know you know that a level tank is a happier tank, and I'm not trying to negate ypur concerns, but an absolutely balls on level tank may not happen...don't over think it, and be patient.
My 180 in my family room is and has been for 3 yrs, off level by about 1/8"....I can live with that...but some people can't or won't...on a recently resealed tank, maybe even more so....I don't recall ever getting a tank 100% dead balls..(Joe Pesci...)