They are not totally identical, but with some small amount of work you can fit a Bristol head to a BMW block.
I suspect the measurement you have asked for will be within about an inch, but note that BMWs tend to have steel sumps whereas Bristol engines have alloy sumps, and the depth of the sump may be different. The pre-war Solex carbs I think are slightly shorter than the post war ones used by Bristols. There are also differences in the air filters fitted on top of the carbs....
Hope this helps.
Andrew B.
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