Milling the base, or the 3rd datum!
To do this, I needed to rotate the mill's turret, and extend the ram so there was enough travel for the cutter to pass over the entire datum surface. Next I placed my big angle plate on the mill, and bolted some toe-clamps on the side to use as stops. Now the weldment can nest in the corner of the angle plate, squaring both of the previously machined datum's. Once the part & angle plate were in the optimal position on the table, I had to remove them and tram the head in its new location!
After tramming, I put the angle plate back on location, and used a big square to align it on the table. Then I placed the 3" face-mill in the spindle, and took several light cuts until I had 100% surface cleanup.
Now I have to tear down the entire setup, and put it all on the other side of the mill's table! Because the long tube sticks down under the table, I have to be careful not to feed the weldment into the DRO's linear scales mounted to the side of the mill! Again, I have to tram the head at he spindle's new location, then I can proceed to milling the base of the 2nd weldment!
So far everything has gone perfectly, as all three datum's on both weldments have cleaned up 100%
Now I can drill & ream all the holes with accurate locations!