I don't believe that you can swap the oil pumps between the M42 and M44. IIRC the M42 gears are 8mm thick and the M44 gears are 10mm thick.
I am also unsure if the later style metal gasket will work with the M42 case (either version). You should dry-align them and visually verify that the oil passages align.
Don't forget to pack the oil pump gears with some heavy grease. Although it is unlikely, you could have serious trouble getting the oil pump to prime if you have had it open and cleaned it out...you need a little "something" in there to fill up the air gaps between the teeth to get it to generate the required suction to get going. You can probably ask the local parts store guy about which type to use...I assume that it just needs to be petroleum based.
Lastly, when it comes to installing assemblies, here's my recommendation for how to do it so that the gasket is not crushed and the parts don't fracture. The red stuff is gasket sealer (not always used, just shown for reference), the green is the gasket and gray are the parts being assembled. I had a thermostat housing break on me precisely because of this many years ago.
Speaking of thermostat housings...make sure that the bypass groove between the two chambers is clear. It makes bleeding the system a LOT easier!
