Hi Doug.
The crossmember is meant to fit hard up against the rear floor timber. It could be mounted further forward on cars with this fuel tank but not on later cars with much less space.
The six leaf springs make the bottom plate much lower than the four leaf springs. Static test shows minimal clearance so I think it will be fine.
If it knocks, I'll insert some spacers between the chassis and crossmember and file a bit away from the crossmember at the point of contact.
