Get a variety of different fits and sizes to have on hand to use on your different engines. The different varieties of rubber have different durometers and some compress easier than others. If any doubt about fit or in an emergency case just wipe a small bit of silicone around the ring to take up any gaps. I have made paper gaskets before to slip onto the venturi first before the o-ring to help take up some slop if I think it needs it. Viton and the red silicone high temp rings are the best I think if you have a source locally for them that you can go to in order to test fit them.
Type at you later,
Dan McEntee