I was dubious about the Blue Anchor SB model as well.
In order to get the 'right' SB type you need to consider not only the size of frame which it needs to accommodate, but also when, and by whom, the SB was erected in the first place. No good having an early style of box if the design was superseded long before your railway is assumed to have been opened. On the other hand, if you have an 'early-ish' railway, then if you're going to use a box like BA (opened 1904) you will need an ''excuse' for it being of a latter type. EG at BA the 1904 replaced an older box when the passing-loop was added. Other excuses might include station layout expansion requiring a bigger box in a different location or a box getting burnt down and needing to be replaced etc. My 'gut feeling' is that a smaller, older, wooden superstructure type on the platform /might/ be more suitable, but it would depend upon the actual layout.