I've been reading this thread with great interest, as I am planning a trans-pacific cruise and was evaluating what was available in my prefered budget. Those whom are contemplating a similar voyage with an older boat should recognize that you'll invest 1.5 to 2 times the cost of the boat in alterations and equipment additions.
One of my previous boats was a 1976 Cal 3-30. What a great boat! In a comparison with the Cal 31, I note that the chainplates of the 3-30 were attached to knees, which went to the keel, whereas the 31 has bulkhead mounted chain plates. The difference between the two rig systems is significant as, as was mentioned earlier, the bulkheads aren't always bonded to the hull sufficiently. It is imperative that they are, but access to assess this? The hatch sizes can be altered, I did this with my companionway hatch.
The Cal 3-30 had a quarter berth & nav. station and I installed a lee cloth on the berth to "tighten up" the space in bad weather. To be honest, I prefer a boat with qtr. berths both port & starboard. Not owning a Cal 31, I have no idea of the space to add tankage, but I added two 45 gallon water tanks under the v-berth & fuel tankage(+20gallons) aft. My 3-30 had a very thick hull, 1.75" where I put in a through-hull. I had the ability to run a cutter rig on my boat - great set-up for offshore - and running backstays.
Really, the only issues that concerned me with my 3-30 were that there was no skeg in front of the rudder, it had a gas(universal) engine & the stove was CNG(very safe, but hard to find in some areas).
It is not of value to consider a boat's offshore abilities in terms of size, other than for consideration of comfort. The Contessa 26, is a typical Carl Alberg design, therefore, very capable of offshore. On the other hand, it's cabin is a miserable hole in which to live for extended periods & only tolerable for more than a week if solo-sailing(my first boat was a Contessa 26).
So, I'd recommend any of the 30 foot Cals - Cal30, 2-30, 3-30 - over the Cal 31. Just one man's opinion, however. I hope this helps & that I haven't offended anyone, as I think all Cals are terrific boats!