I had the same idea (x-rings) and tried to find these in the right size, but no joy there...at least I couldn't find them in the right center section size.

Honestly I'm pretty certain only the dreamers at GM thought these carburetors would still be in use in 2018 and that anyone of a sane mind would be trying to solve these (known) problems.

What I'm doing now is checking the 5 carburetors I have to see when (from a vacuum standpoint) the power piston lifts off the power valve. As long as the piston will lift off and stay off at a point less than 10 inches, I think it will be ok. If any of these pistons won't lift off until after 10 inches, then it is back to the drawing board. The power valve should be definitely closed while cruising and while this may not be a problem at sea level, at 6000 feet somebody might only have 10 or 11 inches at cruise speed. What I'm saying is nobody wants the piston needle to be bouncing up and down on the valve ball and making it open/close at cruise speed. If that's happening, the gas mileage will go to pot. Actually if you stop and consider this, there is a fine line anyway. Rochester from all the data I've been able to dig up designed these so that the piston would be forced down (by the spring, naturally) at roughly 9 inches. And that applies to the B, BC and BV. I'm positive they had a reason for this, but 9 inches seems on the low side to me. So, when your vacuum drops to 9 inches or less, your power system will be in play and that will happen either at high speed or under load. I think I would have leaned toward different margins. I think I'd have made that power system open around 6 or possibly 7 inches and close up around 8 or 9...and with a weaker spring we could do that. Doing that would mean the power system would only open when you really needed it and it would be disabled sooner to return you to the "economy" setting as controlled by the main metering jet. And it was interesting to me to find springs of different strengths in 3 of the 5 carburetors I have here. One of them is notably weaker and I don't know what we have going on there. I suspect somebody tinkered with it along the way, because it is the same length. But I should tell you that spring was in the newest of all the ones I had.

This whole project reminds me of watchmaking (which I began learning in 1958 and now do only occasionally). Sometimes you have to work for a while and then walk away.
Meditate, pray, think, drink a beer, read a good book, cup of tea, whatever. And when your mind is uncluttered other solutions will come to you unexpectedly. When I began this project I decided on an amount of $$ I could spend (some might say squander) and I'm about to the end of that now. Interesting you've mentioned the nitrile compound isn't compatible with ethanol gas. You might want to check that...I found lots of conflicting data out there and at this point it seems the overriding thought is buna-n or nitrile is happily compatible with up to E85 gas. I've noticed no reaction to it here. Viton is fine but not available in the right size, neoprene is in this case not suitable because once exposed to ethanol and allowed to dry begins to become brittle and polypropylene is another one of those compounds you hear conflicting data about.

~ Jon
1952 1/2 ton with 1959 235 | T5 with 3.07 rear end