I did precisely this conversion this past autumn. Took an RX100 7-speed drivetrain, swapped out the rear wheel (hub totally crapped out on mile 3 of an 87 mile ride... but I still finished - quite the workout), so I got new set of Shimano RH550 wheels, chain, cassette, Tiagra STI levers, and because it was cheap, a new 105 RD. Kept the FD and cranks (no problem with the spacing at all).

I had no trouble squeezing the new wheel into the narrower dropouts. It's a bit snug, but not terribly to squeeze in by hand - even when fixing flats on the side of the road. Though it's a steel frame.