There is not really much to add to the really good advice here, but I will try!
Spacers are a good idea, but if you are patient and diligent, there is another avenue available to you. Every now and then, there is a seller on a popular auction site that sells a custom machined tool head for Dillons, I picked one up. The raised portion of the tool head that engages the press is offset and comes with a spacer. Cost was about $30. Place the spacer above the projection, then set your dies for 38 Specials. When you want to load 357s, just put the spacer below the projection. All you will have to adjust is your powder charge. I tend to use Bullseye for my 38 wadcutters, 231 for my 38 SWC, and looking at Unique for my cast gas checked SWC in 357.
Spacers are a good idea, but if you are patient and diligent, there is another avenue available to you. Every now and then, there is a seller on a popular auction site that sells a custom machined tool head for Dillons, I picked one up. The raised portion of the tool head that engages the press is offset and comes with a spacer. Cost was about $30. Place the spacer above the projection, then set your dies for 38 Specials. When you want to load 357s, just put the spacer below the projection. All you will have to adjust is your powder charge. I tend to use Bullseye for my 38 wadcutters, 231 for my 38 SWC, and looking at Unique for my cast gas checked SWC in 357.