JOhn Deere 2010 Diesel Timing- D/C or 26? retarded?

I"m rebuilding a 1964/1965 (ish) John Deere 2010 Diesel. The tractor was recieved with the roosamaster injection pump (DBGVC429-2DH) loosely installed in the crank case, not timed. I had the pump rebuilt, they checked the correct timing marks, and also confirmed functionality on a calibration test stand. My John Deere manual specifies that timing needs to be set with the flywheel at TDC (D/C mark) but when the flywheel was at D/C; I did not have enough travel in the injection pump housing to align the timing marks on the pump. Assuming that someone had incorrectly installed the oil pump drive shaft a tooth off, I moved the oil pump gear one tooth, but still could not quite get the marks to align on the pump due to not enough travel on the injection pump housing. with the flywheel at D/C; it seemed that I was just a few degrees off where I needed to be. I called the pump rebuilder to confirm that he had checked the timing, and described my problem. He stated, that this pump was not designed to be timed off of D/C; but should be timed ~12 degrees retarded, he stated I should check the flywheel for a second retarded mark. I did, and found a 26? retarded mark, but according to my manual, this mark is for timing a gasoline engine! The funny thing is, using this -26? timing mark on the flywheel, the injection pump sits almost center on travel, and I could get the marks aligned on the pump. The engine starts up with a bit of turning over and seems to run pretty good (I think I need a new starter, seems to turn slower than needed, need to hook the new battery up to an additional 200amp charger to get it to fire up-but this is a separate issue). Has anyone ran into this timing issue on the late 2010 Diesel, and can confirm that I should be using the -26? retarded mark on the flywheel for this engine and pump combination?