Building a Cyclocross bike with a MTB Frame...

[ultra-g]

... is it possible to put 700c wheels on a mountain bike frame?

[Lectron]

Very often yes, but you'll have a problem with canties/v-brakes. You'll have to stick to disc brakes.

[velocipedio]

you can run 26" wheels. i've seen frankenbikes like this at the races, particularly for maller riders -- women and youths.

[jeff williams]

I just tried a 700 Mavic with IRC XC tires on my rigid mtb.
5mm clearance under the bridge. I have a Shimano 600 bridge mount brake, maybe I'll see how off it'll be.
Are you going to cut off the canti mounts?
How about moving them up?? Impossible? no..hard yes.

[jeff williams]

The brake mounted with the pads perfect >inline with the rim! I dislike skinny rims, but LOVE light bikes.
I'm horrified and delighted! :( :D

Now the rear brake is different, where are the frame braces? The top one probably wont be drilled horizontal, it will be vertical, one side.

Maybe..if the rear strut is the right distance to mount the tire..and in the right position to mount the brake.
You might drill the frame brace horizontal and tap screw a mount holes for the brake.

Maybe this is a bad idea....stress on the brace. If you did some brazing you could fill the one hole.