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What size expansion tank are you all running? I have a 302 with a 2 core radiator and am looking at a 20 oz tank from Speedway. It's 13 inches tall and I think that would do the job.
I built my own years ago from some 2" PVC tubing. Been running for close to 10 years now without a problem. I keep meaning to replace it with one of those purty stainless once but I never seem to get around to it.
My gut feeling would be to go with as "big as will fit" in the space you have.