1) The difference in dimensions from .38 Special to .357 Magnum (and the only difference) is 0.135" case length.
2) Rather than adjusting and readjusting dies (a sure way to make a mistake), try this:
Find someone who is a good machinist who can make you a washer/spacer that is exactly 0.135" thick and has an ID that will pass the die body. Now, adjust your sizer for FL sizing (i.e., raise the ram up, screw the die down to contact, if (and only if) non-carbide, lower the ram and screw the die in a further 1/4 turn, and lock the ring.
Now, put in the belling die, insert a sized .38 Special case, and adjust the die for the proper amount of belling. Lock the ring. Use the die like this for belling .38s. For .357s, use the die with the same setting, but put the spacer under the lock ring before screwing the die down.
Ditto for the seater.
I've done it this way for umpteen years. I have another spacer, 0.125" thick, that does the same thing for .44 Special and .44 Magnum.