Hi, Bridgestone BT45 are the ticket, in my opinion. They come in many of the older sizes, are easy to find, and you may be able to get them cheap locally during some late winter/early spring sales. I ran them on my 73 RD350 both in Calgary and Stratotech and never had a problem.
Cheers, and good luck, I hope to see a vintage bike out there again!
Thorsten