Your boat is probably quite similar to mine. I have a 1982 Seafarer 26. Like you, I have had a difficult time finding detailed information about the boat. I found the McCurdy and Rhodes NA website and actually talked to Ian McCurdy, who says that a number of the original drawings are still in his archive. It would be great if this information could be made available to owners. Below is a link to M&R.
There are a few owners websites and photo albums out there but not many. Check out Todd's site if you haven't already seen it. Seafarer 26 Ker-Plunk
I bought my boat in March of 07 and have been replacing parts since then. So far I have replaced the sails, standing rigging, lifelines, fuel tank, hoses, thru-hulls,and other things. I have the old fixed ports removed and hope to replace them with opening ports. I am having a hard time finding exact size replacements for large ones though.
My boat has the Yanmar 1GM engine. I have the original manual and can send you copies if you have the same engine. You might try boatdiesel at the link below for additional information - there is a Yanmar forum.
Good luck, Dave
McCurdy & Rhodes - Naval Architects