We've thought about trying to use a DB4 head in a DB2 pump, but since the DB4 head operates opposite rotation of the db2 and the cam ring is different.
If you want to get some wild flow out of a db2 you can also increase the number of charging ports on the head. Bump it up to 6 or 8 so you can get a complete fill at higher RPMs.
Another area to look at is the plungers. Stock 6.9 pumps have .290" plungers installed, stock 7.3 pumps have .310" plungers. We can get almost 120cc out of a .310" pump. We've also built pumps for drag applications running .400" plungers.
If you want to start building pumps (or even building your own pump) you'll have to have a flow bench to make sure the thing is timed, flowing, phased, etc. correctly. Even then, some pumps take 2, 3, or more times going back on the bench before everything is where we're happy. I doubt a lot of guys are going to want to drop 50-80k into a mcm or hartridge flow bench to build a couple $450 pumps.