SpaceX has released a user guide which is only version one and dated March 2020 but lists the planned mounting arrangement as
As a baseline, Starship is compatible with heritage Falcon 937-mm, 1194-mm, 1666-mm and 2624-mm clampband interface requirements, including the ability to host multiple payloads side by side given the large diameter available.
These are described in more detail in the Falcon 9 guide as
The standard mechanical interface between SpaceX-provided Falcon launch vehicle hardware and customer-provided hardware is a 1575-mm (62.01 in.) diameter bolted interface, at the forward end of the launch vehicle payload attach fitting. This interface is designed to conform to the EELV 1575-mm (62.01 in.) diameter medium payload class mechanical interface defined in the EELV Standard Interface Specification Rev. C June 2017.
The load bearing capability of a 62 or 103 inch EELV does not appear to be easily located online but looking at the bolt hole patterns could be quite substantial without issue noting intention for vertical integration with the vehicle.