From memory the existing single coil is quite a large unit. The ALNICO pole pieces were about the same size as Seymour Duncan use for their QP pickups.
The pickup is mounted on a fiber board caddy, as are most Ric replacement bridge pickups. Should the one you choose not fit it is a simple matter to trim the fiber board and bore new screw holes.
Neck pickups mount through the scratchplate, the opening may or may not need enlarged.
It is standard practice, not just for SD, to mount a hotter pickup at the bridge. There is more string movement (energy generated) at the neck than at the bridge, hotter bridge pickup compensates for this.
Personally I am not a fan of Duncan bass pickups. Like Nescafe Instant, being a well known brand does not mean it is good only it has better marketing
If I was re-visiting the RB-414 and planned a pickup swap, Mr Konig would be getting my money : great:
Now I think on it , Konig has had plenty of my money over the years : D