Drop me an e-mail at firstname.lastname@example.org
and lets talk about the 257 AM in your receiver!!!
I have read about this little round and from what I hear it is a nice round and very easy to load and generally quite accurate.
IF this is nothing more then a 308 necked down to 25 caliber I would personally just use starting loads for the 260 Rem with same bullet weights. There is only 0.007" difference in bullet diameter and if your using same bullet weights, with starting loads you should be well under top loads.
As far as dies go if you do not have any, I would get a Redding 260 Rem bushing neck sizing die with the appropriate sized bushing for the 257 diameter neck to size your necks and then for full length sizing when you needed to, I would get a standard Full Length die for the 260 Rem. Basically this would be nothing more then a Body die for the 25 wildcat.
This is only if the case design is the same as the parent 308 of course!!
Get ahold of me on the 257 idea, I have some barrels in stock ready to roll for that one!!