The size issue is why I converted my Sherwood to Magnoval sockets in order to accept an EH7868. I wasnt interested in the prices of NOS 7591, and new ones would not fit. The only practical choice was changing the worn out sockets for something else. I was able to find 4 NOS Cinch (or was it Amphenol ? I forget) sockets for a reasonable price and I got a matched quad of EH7868's for around a C note. It works just fine and sounds excellent, and it cost me about half what I'd have been into it for NOS 7591 tubes. If not for the fact that new 7591's didn't fit, I'd have probably not bothered changing the sockets though.
The 6GM5 isn't currently made as far as I know. Its also got a very small 9 pin miniature base. Its like a fat girl in stilletos. Not very solid feeling in the socket at all. The sockets are also small enough that they won't swap for an octal easily. The Magnovals and Novar will. Magnoval is new EH7868, Novar is old stock 7868. The pins are fatter on the Magnoval base, but you can force them into a Novar, especially an old tired Novar.