I don't want to stop anyone from playing Frankenstein builder, it's fun to play and real advancements can come from it. But understand how big the fight is before you throw the first punch.
You can't compare the efi in gas systems because you can start pumping fuel into the cylinder from the time the time the exhaust valve closes up until the ignition event which is the spark plug firing. Thats nitromethane process- on the edge of hydrostatic lockup. But you have a spark plug to control when the burn starts.
Diesel starts at compression ignition point, so you don't have the control. That's why it's timed with the fuel getting sprayed into the cylinder after the compression ignition point is already achieved, you have to time the ignition. You can't run constant pressure to a "dumb injector". It will never open to let fuel in.
Volumetric Effencicy in TD engines are roughly figured at 3:1. But if you are running a big turbo or two, maybe higher compression, improved flow, etc you can push to 4:1. That means at peak your 400 psi you generate can be at 1,600 psi. so your opening pressure of the injector has to be above that to work at all. 6.5 probably peak in the 3.5:1 area.
If you run your pop tester in a pressurized chamber, you'll see your true injection pattern, much different than on the bench in open atmosphere. Keep that in mind when determining your numbers.
Sounds like you basically are looking for a larger IP. The electronics added to this base design IP was never intended as an improvement. It generates less power, not more. It it less fuel efficient, not more. The only reason the ds4 was created was to meet the obd2 regulations that said fuel volume and timing events must be able to be monitored by computer. Not better, just monitored. Keep in mind the Honda carburetor the rest of the world got to use on gas engines that had more power, efficiency, 1/2 less pollution. They fought to use it here and was shut down because there was no ability to monitor it by the gubmint kompuerz.
Any changes you make to the system would be shot down by emissions standards anyways, so just switch to a big db2. You can go to twin IP set up. Gm in its place and the ford in front for the reverse rotation. The Ford needs to feed the factory injectors because it can push more fuel, and the GM IP pushes it's fuel through the smaller tipped stream injector that spray through the glow plug holes.
Really with the fuel you can run a fully built db2 and use of propane as supplement fuel and nitrous so you can burn it all in time before the exhaust valve gets too far open. Just start with a very stoutly reinforced bottom end.
Here is another option. The old fords HEUI don't get all there pressure from the ip, the engine uses a secondary oil pump to amplify the pressure in the injector. If you switch to those injectors, run a common rail fuel pump to bring them the diesel set at a pressure below pop pressure, then use the ds4 or db2 to pump oil to the injectors oil inlet, timed so the oil increased pressure causes the injection event when you need it, Maybe.
How much more fuel can the HEUI injectors move in the same amount of time? Engine oil aerates way worse than diesel fuel so that will mess with the IP contoroling the timing. Another can of worms.
You can come up with a system to do it, Infact many ways to do it- 8 linear fuel pump instead of 1 rotory and feed cps signal to control timing of them.
Look into some of the IP used in big rigs, you'll find a couple that are adaptable.
My question is -Why? There are pump builders that can get more out of a db2 than an 7.5 or 8 liter engine can swallow. The $ you spend doing it is easier spent on known technology to get enough power to blow up any 6.5 including the p400.
Heavily built db2 will still last 200,000 miles.
