I think you have two types of engine here; "smaller" one that is "D" of 1917 and "bigger" that is likely "E" for 1918 as determined above by engine #. I would think the later is overall longer due to bigger bore, so crankcase, crankshaft and block are longer and simply does not interchange with the earlier type and (I guess) also do not fit into earlier chassis.
The 1917 D block also has water pump entry between #3 and #4 cylinder, while later has the entry between #2 an #3 cylinder. Also I believe that 1917 block has all 12 valve nests the same size and in line arrangement, while the later has one set of valves bigger (not sure if this inlet or exhaust from top of my head) and "zig-zag" arrangement due to this size difference.
And yes, I would also go as much original as possible as Larry suggests, you have great selection of parts to start with, so why to re-invent the wheel 🙂