I don't have the full logs - Prism deleted them, sadly. I do have the lines that showed up right as I hit assemble, and the game froze, though.

[13:13:01] [Render thread/WARN] [me.je.co.gu.GuiEventHandler/]: GUI did not draw the dark background layer behind itself, this may result in display issues: org.valkyrienskies.eureka.gui.shiphelm.ShipHelmScreen
[13:04:15] [Physics thread/WARN] [or.va.co.im.pi.VSGamePipelineStage/]: Too many physics frames in the physics frame queue. Is the game stage broken?

I originally thought this was because of a few blocks connecting it to the terrain, but I removed those, and they didn't fix the issue. I had a few Spruce Planks Steep Top Roofs (from Macaw's Roofs) on the thing I was trying to assemble, and after removing those, it worked fine. I could place them after it was assembled, but not before assembly.