I have to design a storage tank for high temperature service with 550 deg.F.

As I know API 650/620 cannot be applied. So, I wonder about the possibility of
base plate calculation using PV Elite with ASME VIII-1.

Please advice me an applicable solution.

It is true that you exceed the provisions of Annex M of API 650 (M.1.1 limits the maximum design temperature at 500 F).

A pressure vessel and a tank have complete different basic hypotheses in calculation. For pressure vessels you'll never find a behavior as for junction shell-annular bottom in tanks.

Annex M tries to predict how the expansion of bottom will be and this aspect has consequences in how much stress you'll have in junction and around. For 550 F temperature, these assumptions remain the same as for 500 F, the matter is rather on material properties (IMO). And, of course, you cannot argue that you followed a standard- and this fact has legal consequences, I mean the responsibility is totally yours.