As you found, the DB2 has a completely different fuel inlet than the DS4. Depending on how much work you want to do, there are several things you can do to increase fuel supply.
The fuel inlet on the head of the DB2 is not real large, so that is a limiting factor right there. I have been told that certain PowerStrokes had DB2s with larger fuel line openings and these can be modified to fit the 6.5 DB2. I did not go this route, so you are on your own there. However, I can tell you that a large plunger Marine DB2 has the same size fuel inlet as our trucks, so maybe its not that big of a deal?
On my engine, I drilled out and tapped my fuel manager and installed a 3/8" barb fitting. Originally I wanted to use an AN fitting, but there is very little room under the manifold from the FM to the injection pump, so a barb fitting allowed more clearance. On the injection pump end, I installed a matching 3/8" hose fitting. If my notes are correct, the DB2 uses a 7/16" X 24 inverted flare fuel fitting. The ID of this fitting was slightly smaller than the drilled inlet on the DB2, so I drilled out the ID of the fitting to match. It wasn't much, but I wanted the pump to be the restriction in the fuel supply, not the supply. I then used 3/8" fuel line from the FM to the pump. I figure this is about the best you can get without modifying the pump itself. If you look at the stock fuel inlet barb fitting for the DB2, its pretty small. A 3/8 hose will give you more fuel flow to the pump, so it should help.
This is my Fuel Manager. I don't have any pictures of the pump fitting.