In theory, could a P7100 pump from a six cylinder cummins be fitted to either of the six cylinder engines?

The firing order might not be the same, but with some creative line routing it might be solved. So long as the orbit of the cam lobes coincide with the combustion cycles on the engine.

Are there physical constraints? Is the P pump too large?

All the fuel we would ever need with that bad boy.

__________________
Vehicles:
2002 SLK 230
Gone but not forgotten:
1983 300D
1981 El Camino 'OILBRNR' - 6.2L diesel
OM617 powered '86 F150
1984 BMW 524td
2001 VW Beetle TDI
1994 Sunurban 4x4 6.5L diesel