The GP's pulling voltage down that much is probably a positive sign if anything -- suggests that at least a good majority of them are working, assuming no issues elsewhere in the circuit. If you're concerned, you can check voltage drop at the GP bus vs. at the battery positive terminal when the plugs are energized; under normal circumstances, with 6 healthy plugs, total system voltage will drop when the GP's are energized (enough so that you should be able to hear the pitch of the door-open chime change) but excessive voltage drop isolated in that supply circuit may cause problems. If you're not happy with your measurements look for loose/corroded connections at the GP relay/batt + terminal/GP bus... I suspect it is unlikely you will find issues here, though, and in any case probably not at the root of its failure to start...
Do you know for certain whether this engine has ever run since the new timing belts and injection pump were installed? It is possible that the problem is a loss of prime and you're headed down the right track already, but when you see an otherwise nice car get put away immediately after having a lot of money invested in it, often raises a big red flag about the new parts that were installed ---- specifically, suggests the likelihood that whoever put those new parts on never succeeded in getting it going afterwards to begin with.
This is something you see fairly regularly with these cars, more than the average, due to their rarity and some of their unusual service requirements. The injection pump and timing belts on these motors are not parts that can simply be "replaced" in the sense of just taking the old one off and putting a new one on; since they affect the engine's crank-to-cam and injection timing, factors which are highly sensitive on a diesel engine, they have to be put together with precise methods and specifications, using an extensive set of special service tools. Even a small error can make the engine run poorly or not run at all. If the job is attempted without the correct tools and techniques, a gross error is virtually guaranteed, and engine damage is possible as well. Unless you know that the engine and fuel system work was done by someone equipped with *all* of the necessary special tools/skills/knowledge, I would not operate under the assumption that the engine is set up to run as-is other than needing to get fuel to it. In particular injection pump timing being off by more than just a couple degrees will not only prevent it from starting, but in some cases make it act just like there's no fuel getting to it -- sometimes they won't even make any smoke.
Agree regardless that first thing should be to ensure that the fuel system is bled all the way to the injectors; as stated, this can be surprisingly difficult and take a lot of time, and is aided massively by putting some kind of low-pressure pump in line before the fuel filter to force-feed the IP some fuel. Electric pump or manual hand primer bulb both work fine; all 6 injector union nuts loose; crank motor with foot to the floor while simultaneously running your electric or manual booster pump; as soon as you see fuel running out of all 6 you're done. (I often also disable the glow system by pulling the small black ground wire [spade terminal] off the bottom of the relay while doing this so that the starter spins slightly faster and to avoid unnecessary wear+tear on the plugs.) Once bled, tighten injector unions, reconnect GP relay if you disconnected it, and see if it starts. If it does not fire off almost immediately, then don't keep cranking it, you probably have more work to do.
If you get to this point, some further investigation of the car's history and who has been involved with it in the past, if possible, may be your most efficient first step towards getting it figured out. Who did the work, and what were their qualifications in terms of abilities and equipment. Was it ever on the road again after the last time it was in the shop? If you're getting fuel at the injectors and voltage at the plugs but have no smoke or fire, then unless you can verify beyond any doubt that it was set up correctly by whoever worked on it before, you need to go back to the beginning and check/correct the engine's basic timing setting. (Not hard to do -- the manuals you got may describe it, and if not you'll find plenty of help and advice here.)
One important tip: under no circumstances should you resort to starting fluid. Many mechanics are tempted to fire the motor using ether as a "clever trick" to get an uncooperative fuel system primed, but in the case of one of these, it WILL cause instantaneous and terminal engine damage, and you don't need it anyway..... if it needs primed you can do that easily on the starter, and if the timing is off then no amount of ether will get it to stay running no matter what until the timing is corrected. Don't turn to the bottle, you'll regret it if you do!
Congrats, sounds like a great find -- bet it will be a sweet runner once you get it sorted out!