You should be able to grind down a socket, both by tapering the top and shortening the bottom so that it will fit in.
Just checking - do you have original Bristol studs or later replacements? All those I have spoken to recently who regularly rebuild these engines suggest that the original studs were not up to much and will stretch.
Also ensure you have good quality washers about twice the thickness used elsewhere on the engine.
The replacement studs with nuts supplied by TT Workshops years ago were 9/16" across the flats so required a bit of DIY to make up the tools.
The spanner not only required a bit of encouragement from the oxy acetylene torch but also the ring end needed to be ground a little around the periphery.
https://www.bristolcars.info/forums/...1&d=1760365462
https://www.bristolcars.info/forums/...1&d=1760365462
https://www.bristolcars.info/forums/...1&d=1760365462