BEFORE YOU DO ANYTHING!!!
Grab a pair of calipers and a straight edge rule...
Run the straight across the lock nut face...
Measure distance from freewheel stop on the hub to the end of the lock nut (represented by the edge of the rule.)
Turn axle 180 and repeat...just to be sure...
If that distance is at a minimum 36mm - you are qualified for a "road" 7-speed freewheel (Shimano MF-HG-37-7)
If that distance is at a minimum 39mm - you are qualified for a "mtb" 7-speed freewheel (Shimano MF-HG-40-7)
Chances are pretty good you'll find your measurement to come out to be 37mm - and that you're already good for road 7-speed. Just make certain you buy the correct freewheel.
=8-)
__________________
5000+ wheels built since 1984...
Disclaimer:
1. I do not claim to be an expert in bicycle mechanics despite my experience.
2. I like anyone will comment in other areas.
3. I do not own the preexisting concepts of DISH and ERD.
4. I will provide information as I always have to others that I believe will help them protect themselves from unscrupulous mechanics.
5. My all time favorite book is:
Kahane, Howard. Logic and Contemporary Rhetoric: The Use of Reason in Everyday Life