I have a Delaval 73, and I would say if you can't hold your hand on the pump it is too hot. I run a fan on mine, I'm pulling 23" and I can hold my hand on to pump. You could try different oil to see it the pump will run cooler. I don't know about a bigger tank, it should help. I have heard that you have to have vanes made for this pump but I have never looked for them myself. I think the 3 main things to check is oil, heat, and keeping sap out the pump.
Matt
2x6 Phaneuf raised flue pans
homemade oil burning arch & hood
homemade RO,auto draw-off, releaser,auto-canning valve, filter press, ext.....
850 taps