Curt,
Are they 1157 & 1158, or 1167 & 1168?
There is good information on the Pertronix site - they have a catalog that can be downloaded. However, they do list 261s in earlier years and 216 in later years.
Here is the Pertinent Page (PDF). Here is a JPEG of the pertinent page The 1953 and earlier distributors need a specific style/size unit and the 1954 and later distributors use a different style/size unit. The footnote explains how to specify positive or negative ground and 6v or 12v.
Will you be using the 1953 distributor (1167 Pertronix unit) or the 1956 distributor in the 261 (1168 Pertronix unit)?
To complicate the answer to your question, I think that some people have posted that 1953 was a transition year for the distributor. If you have both distributors, see if you can discern any differences.
Tim
