1990 360 engine, trying to retrofit it to use a mechanical fuel pump.

I forgot about that thread!
The engine was thought to be an '89 but the casting date reads XX-XX-90. It is an LA series of course. I thought it was a flat tappet engine but it is not.
I had much of that story wrong.

Quoted from that thread of mine that you linked, authored by "Professor Fate":

Roller LAs (from the factory) all had long snout cams. Some passcar rollers (non-TBI, with feedback carbs- usually found in Fifth Avenues and the like) already had mechanical fuel pumps. The later '88-'91/'92 TBI motors (found in trucks and vans) used an electric fuel pump in the tank and did not have the mechanical pump, however they still retained the long nose cam. The timing cover either used a block-off plate or did not have the pump boss finish machined, and did not have an eccentric installed on the cam- there was a special deeper cam washer used to make up the difference in depth with the eccentric deleted. Use a standard LA cam bolt washer and the eccentric is a bolt-on; either remove the fuel pump block-off plate or finish opening up the fuel pump boss on the timing cover- depending which you have- and you're good to go (or swap covers if you don't feel like opening up the boss on the TBI cover).
Magnums all had short snout cams and need the Hughes adapter and a timing cover/water pump/accessory drive swap if you want to use a mechanical pump.
Side note: Don't try to use a Magnum cam in a roller LA- it will fit and run, but it's lobes are ground for the Magnum's 1.6:1 rocker ratio and there will be a resultant power loss when run with the LA's 1.5:1 rockers.
Conversely, a factory LA roller cam used in a Magnum can give a nice little low-cost power boost with the Mag's higher ratio rockers.