If your 126 mm frame is aluminum, carbon fiber or held together by glue, you shouldn't try to spread it. That still doesn't mean that you're stuck with 7 speeds!
Any 7-speed Shimano Hyperglide Freehub will actually work with 8 or 9 sprockets, without any modification! What you need to do is to use 8 of the sprockets from a 9-speed cassette, with the 9-speed spacers -- or 9 of the sprockets from a 10-speed cassette. This trick also is useful when cassette body transplantation isn't possible -- on an off-brand hub or a Sachs 3 x 7 hybrid-bearing hub.
To make this work, you'll also need to use a 9- or 10-speed chain and shifters. Your old 7-speed derailer (except 1996 or earlier Dura-Ace) should work OK if it isn't too badly worn. Most, but not all, 9- and 10-speed shifters will work.
The limit stops on the derailer will cause the useless position on the shifter to be locked out, so this will work as a perfectly normal 8- or 9-speed rig.
Reader Justin Mohar writes:
I ran a "9 cogs of a 10 speed" set up for about a year on my road bike (having been originally inspired to give it a shot after reading Sheldon's "8 of 9" article) while waiting to build a new bike with a true 10 speed set up. I used an Ultegra 10 speed cassette (with the 4th cog and one spacer removed) on a 7-speed 105 hub. I also, of course, used 10 speed shifters (105) and chain (Dura Ace). My cranks and derailers were all 105 "7-speed," from the same group as the hub. This setup worked surprisingly well; the only issue was that shifting across the spot where the missing cog should have been was rather slow in both directions. I assume this was because the shift assist ramps on the now-next-to-each-other 3rd and 5th cogs did not line up properly, but I never really confirmed this in any way. Bottom line: it can be done. Feel free to post this info with your next update.
Justin is right about the reason for slow shifting. This issue is more acute with the narrower 10-speed inter-sprocket spacing. If instead, you remove the smallest or largest sprocket, all of the ramps will line up. Most 10-speed cassettes, and the fancier 9-speed cassettes, use a spider assembly for the larger sprockets, and so you would only be able to remove the smallest sprocket. You would need then to replace the second sprocket with one that has a built-in flange.
You could even have all 8 of 8 sprockets on a 7-speed body if you go to the trouble of dishing the inner sprocket -- or even a 9 of 9 or 10 of 10 if you bolt the innermost sprocket to the next smaller one. A dished sprocket was at one time a commercial product available from Tom Ritchie, but this now is a do-it-yourself project. More details are here.
