327amc,
The head casting number also confirms it's a 261 (same casting number used from 1956-1962). On the note you indicated there is not stamped serial number on the machined pad behind the distributor, that is a good indicator that what you have is probably a service replacement engine. They were sold without any serial# stampings.
As 52Carl mentioned above, the 261 does have a slight bump out on the pan gasket set not found in the earlier 216/235 engines. Jon G is also correct that the later 235 and 261 engines used the same part number.
If you take a look at these 3 different vendor ads (and the pictured gasket sets) you can see these sets much more closely match your removed set. 2 of the 3 actually show the same neoprene end pan gaskets pieces.
Jim Carter 261 pan gasket setThe Filling Station 261 pan gasket setEgge Machine Company 261 pan gasket setThese are just 3 of our usual vendors that popped up in my search for the 261 oil pan gasket set. Your note that you will purchase a '56 235 set should be just what you need to fill the need.
Let us know how you parts search turns out.
Dan
