Agree with this.. A sump itself is pretty inexpensive. If your tank is already pre-drilled for the plumbing, then setting it up should be a snap. For what you will spend to cap it off, you can spend a bit more and utilize themWhy don't you utilize them and run a sump with the tank? You could put one together for pretty cheap.
It'll be fine, and it looks like the holes are in the bottom of the tank, so if you put the pvc/cap in from above, not below, the water pressure will actually push everything together tighter instead of trying to force it apart. Besides, properly primed and glued PVC is a whole lot stronger than you'd imagine.but wouldnt the weight of the water break the cap? The water pressure?