Check that the cases are indeed matching. If you look at the bottom of the cases, on one of the ribs for the crankcase bolts, one side or other of the sump plate, are stamped numbers. These are about 1/8" high and are the same on each crankcase half if they match.
The lip on the crankcase mating joint, damaged on your set of cases, is the critical item for ensuring the main bearings are in alignment. The lip sizes vary by a surprisingly large amount from set of cases to set of cases so odd crankcase halves are very far from interchangeable.
36 4G, 37 VH, 54 KH(A), 75 Healey 1000/4, 52/53 ex ISDT KHA (project).
Former Machine Registrar & Archivist, General Secretary and Single Spares Organiser (over a 25 year period).
Now Archivist once more - but not Machine Registrar.