I don't know how easy it would be to find a 261 either, but I understand they were used in 60's Canadian Pontiacs and you might find one of those almost as easy as a 70's 250.
I've been to Surrey but never looked for wrecking yards there so have no idea what might be available.
But if you are going to deal with the conversion issues to go 250 why not consider a 292 and get a bit more torque and power?
I know, you're not there yet. I'll keep watching what you post!
