I believe it was you that posted about the SB dies for the 250 Savage in the collet die column.
I've been looking over some dimensions of calibers trying to find an inexpensive way to SB size the 250 Savage. The max base diameter of that little jewell is .467", compared to the '06 and 308 dims of .470" and the 244/257/7x57 dims of .471".
I think the 308 Marlin express will work. Though I don't know the actual max dimension of the 308 MX, the dies measure about .465" and the sized down 307 cases measure .467". And the 308 MX is .450" at the shoulder, the 250 is .414" so the shoulder would not be sized down, just the base. This would give you an excuse to by another set of dies and it could also double as a small base 250 Sav, maybe. I didn't have any 250 brass here to test but even if it didn't work, you'd have the 308MX dies. Then You'd need to get the gun.
Edit: I just found the dims for the 308 MX and it is also a .470" base. If you have a SB 308 try that and see what happens with it. I think it is .002"-.003" smaller than standard.