I'm thinking about converting my "old" road bike to cyclo-cross. I have two choices as far as rims I already own go, some Weimann 27" (could likely handle up to 1 1/2 to 1 3/4) and Mavic Open 4 CD's (28c likely max). Both sets of rims are clinchers. The frame was built to handle 27"'s, but I've been using the Mavic's without any complications.

Any thoughts on what size, brand and model of tires I should think about running, which rims would be a better choice and what the availability of 27" clincher cyclo-cross tires are?

Go with the 700C wheels. There are no 27" cyclocross tires anymore. I think you are being overly conservative with you tire width. You should be able to get tires marked 700x35 to stay on Open 4 CDs without much trouble/risk.