Your application is not cover by API 650 (or maybe I was not able to understand what it is!). For Sumps, API 650 imposed some distance as is indicated in Table 5-16—Dimensions for Draw-off Sumps. Looking for a 6" nozzle, distance from shell to sump edge is comfortable. This is not really an argument for your application other than API tries to place the "opening in bottom" to a "comfortable" distance from the shell.

What I tried to say is that both shell and annular bottom respond together to the hydrostatic load, thermal, seismic etc loads. Depending on the shell to bottom size connection, this means that the connection would be subject to imposed displacement and rotations and/or it would restrict the annular bottom/shell response to an seismic event.
Especially for seismic calculation, it is difficult to say something without FEA, but even in this case such analysis applied for seismic case should consider all tank and the connection shell- annular(a very expensive and time consuming one). By the other hand, your Client would ask you to extend the analysis for your connection under the provisions of paragraph E.7.3 PIPING FLEXIBILITY from Appendix E of API 650, case in which I consider it would be very difficult to qualify that connection or to provide an alternative calculation without FEA.