OK, you have a progressive press. You cannot buy an extra tool head (like some Dillon models and the Lee Pro-1000)
But you have the bushings.
The spacers are not working for you because you lose your die settings on account of the lock rings moving when you unscrew them to install or remove the spacers?
Simplest solution is to get lock rings that don't move.
http://www.hornady.com/store/Sure-Loc-Lock-Ring-1-Each/
Adjust the dies in the bushings for the 38 Special, tighten the lock rings and never move them again. The spacers should take care of the length difference between the .357 and the 38 special brass. If the spacers are the wrong height, it should be a very small difference. Sanding the spacers thinner would take some time. Shimming them up, you're on your own.then adjust the SPACERS for .357 Magnum. This will take some time, as sandpaper may be involved.
Personally, my solution is to never shoot 38 Special. All my reloads are in .357 brass. I have only ever owned 50 rounds of 38s and that was back in 1975. When I want 38 Special loads, I just load my .357 brass to that power level.
Interjecting my personal feelings on the matter: I think you are going overboard. It isn't that hard to adjust your dies (3-die or 4-die), especially if you make one "dummy" round to use as a template for adjusting your dies.
Make one PERFECT cartridge with no primer and no powder for each brand of brass, style (profile) of bullet and degree of crimp you habitually use. Place that in your press, run the ram all the way up and adjust your seat/crimp die(s).
For the case-mouth belling die the process is similar, but there is no bullet in the case. You just crank the die down until finger pressure isn't enough.
Keep your dummy cases in labelled boxes, padded so the empty case don't get bent.
But that is a lot of work for a marginal convenience.
If you practice adjusting your dies a dozen times it will become second nature to you. And then you will say to yourself, "What was I THINKING?
Lost Sheep