^ I imagine both lots of pickups are passive, and just the Pre is the active part...
1 for 1 wiring swap... presume Pos from each pickup with shared ground...
Are you removing the Power Tap?..
I would try disconnecting the lead first hopefully defaults to std 'bucker.
Mk1 Barts? These are infact a Reverse Split P in a Soapbar not a full width humbucker.
No real need to replace the pre, although you could go passive
Been thinking something similar
Mk1 Bart, Passive V and T... Simple P SR... and cover the bridge cavity and 3x EQ holes.