Use the LDT-465 mark.
I just worked on another friend's IP last weekend; he could get it to start with ether, but it would instantly die as soon as it fell below 1300 rpms. I pulled the hydraulic head to check the fuel button on bottom of quill shaft, and sure enough it was off. The button retainer clip looked fine, but it must have fatigued and fallen off. Once I installed a new button retainer clip, it fired up on its own and almost held its own idle; the more it ran, the more it would run as it was supposed to. As the Seafoam worked its way through the IP and the injectors, less smoke came out of exhaust & the idle smoothed out.
Here's what I would do (if you haven't already): disconnect the top radiator support & radiator hose, loosen the lower radiator mounting bolts as much as you can, and tip radiator forward for clearance to remove the fan. With a large 1-5/8" wrench (radiator shroud usually blocks a socket from being used) you can now bar over the engine. Remove the front valve cover; as you line up the timing mark on harmonic balancer, check for clearance on your #1 valves (intake & exhaust). I've found some to be loose, and others to have no clearance at all....but if you have clearance on one valve, the other should have clearance as well or at least be closed (and allowing you to adjust the proper clearance while you're at it). It's pretty obvious; if you bar the engine over another time to line up initial timing marks, you'll notice that the exhaust valve will open, and you're halfway there. Keep barring engine over (preferably with engine shutoff handle pulled out to prevent accidental starting, better safe than sorry) to line up timing marks on harmonic balancer, and check valve clearances again. You should be where you need to be now.
To the left of the hydraulic head (underneath the fuel return elbows) is a small rectangular port held on by 4 bolts; remove these with a 7/16" wrench and remove rectangular cover. With a flashlight, look into port and you should see another timing needle that hopefully points right at another timing mark. About half the time I pull these covers, they are a couple of degrees off. If the mark is off while the harmonic balancer is properly aligned, then you need to do a fine adjustment to IP timing gear to get it right.
Remove the gear cover on front of engine, in front of where the IP bolts into it engine mount (where you would mount a Vickers pump, for example). You'll see a gear wheel with three slotted mounting holes. Use a couple of 9/16" sockets and handles to loosen these 3 bolts and then turn the larger nut in the middle to properly align the IP timing marks. DO NOT MOVE THE HARMONIC BALANCER TIMING MARKS DURING THIS! Once properly aligned (timed), tighten the 3 bolts and replace the two covers.
Now remove the fuel shutoff assembly housing and the spill port assembly to check the hydraulic head timing....you should see the red tooth one to the right of the pointer; I usually have to use a flashlight even in bright sunlight. If you don't see this, your timing is way off and you need to remove the hydraulic head to get it timed properly (although I've never been able to reassemble it if it's been that far off...YMMV). At this point, I would remove the hydraulic head to check the button retainer on the bottom of the quill shaft; it sounds like it might have fallen off. You've come this far, why not?
3 years ago I bought a batch of 12 button retainers from AMBAC, and it looks like they are still available for purchase. I've just about gone through my entire stock of these already, only have 1 or 2 left. IIRC the part number is RG902, and shipping costs more than a purchase of 10 or 12.
You might also want to pull your injectors and have them tested at a shop to eliminate any potential issues there.
After all you've done, I can't imagine why it wouldn't fire up after completing the final steps for timing & injectors....
