So after putting some miles on it with full doses of lube additive, I'm pretty sure it is making things better, but not actually solving the problem.
I'm getting more smooth drives. And when it is on a smooth drive, it is really nice to drive. And when it does a rough drive, it isn't nearly as annoying as it had been. It's hard to describe what I mean by rough, but it is like all the diesel rumble and noise, but sharp and loud in a way that it doesn't need to be, to the point where my passengers and I can tell the difference between a good drive and a bad drive. It feels/sounds a lot like a bad u-joint, but it is coming from the engine, and despite several attempts in different ways, I can't pinpoint any location.
That said, I think the issue is something to do with air getting in the lines. I have a feeling it may have something to do with the conversion not done from the V8 style pick-up. I'm imagining that the fuel is somehow getting tiny bubbles in it. I could be totally wrong. But that's the message I'm getting from my spiritual connection to the Rover gods.
One way or another I'm awaiting the parts to change the fuel lines and the sender/return. This needs to be done either way, if only to avoid future problems, and to allow me to use the full capacity of the tank without starving of fuel at 1/4 tank. If this doesn't clear up the symptoms, I'll have to get the injectors tested, maybe pull the IP to get it rebuilt, etc...