The arrangement of stops/tabs is an area where national preferences and practice have a large impact. I suggest you look at some of the sample sets available for HW to see the diversity of practice. Hereford, Salisbury and St Anne give the usual "English" arrangements, Rotterdam gives a good view of a modern European organ etc.
The arrangements for pistons is often even less "standardised".
Theatre organ layouts are quite different again.
Translating side stops to tabs can be more challenging too.
One of reasons I like touch screens is that the layout of the stops can vary to match the instrument being played.
Unless you are going to use only one sample set, you need to think how to label physical tabs / stops or use a dynamic labelling arrangement such as e-stops (from Midi Boutique) to reflect the allocation of stops to physical controls.
Iain