excessive headspace

[Jack]

Using 8x57 brass would probably be easiest, since, being the same length as 7x57, you won't have to trim.
Use your 7x57 sizing die, but back it out of the press a few turns from where you usually have it. You should remove the decapping pin/expander ball from the die when you do the initial resizing.
A well lubed 8x57 case should size to 7x57 when sized in the 7x57 sizing die. New brass works best for reforming, BTW.
Now, since you had the sizing die backed out a few turns, the newly formed 7x57 case will have it's shoulder farther forward than normal - that;'s what you want. Wipe the lube off it and try to seat it in your rifle's chamber (gently). If the bolt won't close on the formed case, screw the die down half a turn and size the case again. Try the case in the chamber again.
Repeat this process until your formed case will chamber fully, preferably with just a bit of resistance.
When you've done that with one case, you know where to set the sizing die to make properly headspaced 7x57 brass from 8x57's.
You can then load the reformed cases like you normally would.
I would suggest, tho, that you stay with rather mild loads.
And I'd still have a gunsmith check the rifle out before you do anything.