The pre-'54 gauges are in a round housing and don't bolt in. The '54-up are a differant shape for a reason, they are 12 volt. However, some of the innards are swappable. But, the only one that you even want to swap is the ammeter, since the others are either 6 volt or for the low oil pressure of the 216. Also, the faces have a differant shape, but you can swap those by drilling out the rivets.
I put an oil pressure gauge out of a '54 in my '50 so I could get the 60 lb gauge I needed for my 261. I just swapped the faces, the rest of it bolted right in.
