My suspicion that there was too much bed weight was confirmed when I did the

initial assembly. The 8mm rails did bow too much for use as-is. In fact they bowed so much there was a bed height difference of almost 15 mm from edge to center. Not usable...

So, to rectify, I disassembled again and put the old bed back together. I am printing out new ends and bushings to accommodate a 12mm rod. Probably overkill, but it is what I have right now. These were initially intended for the Z-axis extension, but it will have to wait.

On the up side, I managed to reset the configuration to the larger bed size without too much trouble. First, I did up a custom machine in Cura, specifying the new dimensions. Then using the G-code command line in Cura, I reset the X axis max. Seemed to work without a hitch.

BTW, the green stuff on the bed is 3D-Eez, a surface coating that makes every print stick. I love it.