There's 28mm between the base of the rails and the base of the blocks. 1204 ballbut is 25mm deep, but 1605 is a whooping 40mm, so I'd need 12mm more clearance, it's almost like adding an additional 12mm plate just to get clearance. How do people normally get around this? is it packers / pockets and double plates to get more clearance?

The bolt head can go from the top on both versions of the housing, it's an M4 bolt and I have 8mm clearance on top for the head. Standard head on a caphead M4 is roughly 7.2mm.

Regards,
dsc.