700 wheels on bike made for 27" wheels I just bought an older road bike with 27" wheels.  I am curious if I can mount 700 wheels (typical road size) on it.  Has anyone ever done this?

The main reason I am interested in doing this is so that I can have more tire selection.  I like the body positioning on my road bike but since I live in a rural area I do a fair bit of biking on gravel/dirt roads and pathways.
Yes, this has been done and is very often done for the exact reason you mentioned: far greater tire selection.

In a general sense, the important considerations you must make stem from the difference between a 700c wheel and a 27" wheel. If we look at a more precise measurement than the common nomenclature, the bead seat diameter, the difference is more obvious:


700c wheel: 622mm BSD
27" wheel: 630mm BSD


This means that the former is 8mm shorter in diameter than the latter, and when you mount a 700c wheel to a bicycle with brakes set for a 27" wheel, they will be mis-aligned by 4mm (a significant amount).

Depending on what type and variety brakes you have, you may or may not be able to reposition the brake pads correctly. In the case that you can't, you will be able to replace them with brakes designed to have a longer 'reach'.