From my experience I suggest you keep an eye on your newly installed pump.
Many years ago, I, like you chose to use a resilient material between an undersize replacement pump and the original hoop. About six months later I was changing the fuel filter on this car and noticed that the resilient material was mostly gone and the pumps was being held in place by the tubing attached to it. Curious as to how this might have happened, I started the car and saw that the pump I installed vibrated ever so slightly when it was on, but enough so that over time and many miles its movements tore apart the material. It was so long ago but I think the material was a used piece of butyl rubber that I found in the shop. Since then I've been installing replacement pumps using new hoop that I make to fit the replacement pump. I also place low durometer Viton between a pump and the hoops. For me it is mostly about not having to do a job twice.
Of course I don't know what you used, and I don't know to what extent you pump vibrates nor the frequency of those vibrations, so maybe everything will be fine. But for me in this instance my quick and dirty didn't work.
Last edited by nunki; 06-28-2016 at 11:17 AM.