I plan to make this unit into a 2/10 without the screen updates for Mac compatibility.
Without any other S.R. updates, can the ROM you spoke of be used in my 2/10 setup with an 800k?
Yes, the "XLSSDSM6" I/O Board ROM is for use with either a 400k or 800k drive on a 2/10, and the cabling is the same, but the 400k and 800k drives are different heights so mounting them in the drive cage to align with the slot in the front panel is different.
For both the Lisa 2/10 aka XL and 2/5:
The square pixel screen modification is completely separate from the I/O Board ROM & 3.5" floppy drive options (aside from not being able to combine square pixels and Twiggies with any available operating environment, except BLU maybe?)
Also, MacWorks can be used with or without the square pixel modification (There are variants of MacWorks XL specific to each screen configuration, while MW+/II are universal), and I suspect MW+/II varieties all operate ok with either 400k or 800k floppy drives. MacWorks XL up to 3.0 predate the 800k drive; I don't recall if they work with it 100% or not. Other operating environments (LOS, Workshop, *nix) don't support the square pixel configuration (at least until LOS source can be recompiled).