Replacement shifter for Shimano exage 7-speed - modern shifter compatible? I've got an early 1990s Schwinn Crosscut with Shimano Exage LX400 components.  I've just pulled it out of storage.  The bike is in pretty good shape overall.  The derailleurs are moving freely and the gears and cranks don't have any significant visible wear.  

The front shifter ratchet mechanism was stuck.  I couldn't get in very well but i sprayed it down with WD-40 and tried to work the mechanism, but no luck.  

I replaced it with a Shimano Altus SL-M310.  that is working mostly, I've got it adjusted to the point that  the chain rubs slightly on the front derailleur when on both the smallest crank and rear gear, I can live with that.

Meanwhile, the rear shifter failed while I was testing the new front shifter.  Similar ratchet mechanism stuck.    

I can easily enough (hah) replace the front shifter with the matching right side SL-M310, but with 7 speeds instead of 3 there's a lot less room for error.  Is this new shifter likely to have the same distance between gears as the old derailleur?
For the shifter to match, you need to match the number of speeds, cassette width and shifter actuation ratio. For Shimano everything under 10 speeds (except 8-speed Dura Ace which most people don't have to care about) has the same actuation ratio, so all you need to know is the new shifters are 7-speed Shimano. These can still be bought.