Yup, 42 Amp Alternator (C5AF-10300-B) with the smaller single groove pulley.
Per Bob Mannel's Small Block V8 book, for 1965 there where 2 pulley diameters offered in 65. The larger size, 3.9" and the smaller size, 2.6". I believe the book lists the actual pulley diameter measurement. The book notes that the Ford listed pulley diameters that could be up to 1/4" off when you take an actual measurement of the pulley diameter. As Bob mentioned, the larger pulley was for the HIPO, so yes you want the smaller pulley.
Regarding the smaller pulley, I also noticed from the book that Ford had a running change and used two different alternator pulley, fan and pulley spacer combinations in 1965. Before about 4/1/65 (not an exact cutoff date) the combination was 1/4" thick pulley spacer, flat center area fan and an elevated pulley stamped C5AF-10A352-B. Sometime after 4/1/65 the combination was 5/32" thick pulley spacer, a convex center area fan and a flush mounted pulley stamped with C5AF-10A352-G or C5AF-10A352-K. With either combination (don't mix and match!) the net result was that the pulley belt grove was same distance from the alternator housing. Based on the May, 65 build date of your car, I would think either pulley, fan and spacer combination would be acceptable for All MCA judged classes but I could be wrong since I'm not an MCA judge.
If you don't have a copy of Bob Mannel's book, I highly recommend it.
http://www.fordsmallblock.com/Mark
