TR6 Oil pump drive barely engages

This doesn’t seem right. The slot on the bottom of the distributor shaft only engages the oil pump blade by 3 mm. New oil pump. Distributor never installed since engine rebuild. The oil pump blade is about 8 mm tall.

Have not been able to engage it at all, actually, because I can’t seem to line things up. But, first things first... why is the pump barely engaging and potential causes/solutions? Can I move that shaft further down?

The oil pump shaft is just a pressed (tight) fit into the pump.
I know of a couple of instances where the slot in the distributor drive gear was not lined up with the driven dog on the oil pump shaft.
Bolting the oil pump up forced the shaft down into the pump resulting in what you described
The other scenario was when the oil pump was installed prior to the distributor drive gear...then unknowingly the drive gear missed engagement with the oil pump shaft and when the distributor's pedestal was installed over the drive gear, the misaligned oil pump shaft was driven down deeper into the oil pump, making proper engagement with the drive gear impossible...and to make things worse, the result was no oil pressure when the engine fired up ....And what made diagnosis difficult was the fact that the artificial rotation of the oil pump with an electric drill, did make the oil pump work.
If none of that applies then you may just force the oil pump shaft up higher out of the pump, making longer

Thanks poolboy. I found a post from 2014 where the poster realized that the oil pump drive bush was not seated all the way down. So hammer hammer and I gained 1.10 mm, thus I am now using about half of that oil pump blade. AND I finally got the pedestal to drop all the way down. I did the end gap measurement and I am about .0044 preloaded, so I need a .010 gasket. I guess it could also be the case that the bush needs to be driven in even further. All of this is a preliminary run through because dizzy will not be installed until I'm ready to spin the oil pump (after I drop the body back on the chassis). I'd rather do all this fiddling around before body installed.

In one of my rebuild gasket kits, there a three paper gaskets that seem to fit the pedestal, but combined they only make up .0027 thickness.