Congrats on 2,000.

A 1968 Carlton may well have been built for 27". You really need to know. If you've got even a 700c wheel, stick it in there and see how your available breaks reach. Then measure.

I've recently gone thru a very similar process. I acquired a great '63 frame, built for 27" and fenders. I spent a lot of time exploring a 650B conversion. In the end, however, I've been convinced to go 700c with 35mm tires.

First, I found that even 700c wheels required long reach brakes. I've gone with the R559. But, skipping many other details, the final straw was Chris K, the owner of V-O, who looked at me and said, "Why would you do that? The handling will be much better with the 700c and you can run as wide a tire as you want."

Heck, I went to him for validation. I got common sense.

That said, I still look longingly at those Grand Bois 42mm tires... sigh.