Thanks for the measurement, just checked mine and they are close to 1 3/4' with the studs bottomed out in the barrel. If they were 1 5/8" they would only pick up a few threads, I think these are the original heads and barrels together with studs. Last time I had the head off was in the 1970s when I holed the piston during a race and then I probably had the engine out for a rebuild.
Being an early post round barrel VCH things may not have been standardized at that point prior to the 1952 builds, looks like I will just drop the engine down a tad.
1951-52 VCH under restoration