DonT,
Since you have a lathe, it would be little trouble to make up a neck-sizing only die for that 8mm Siamese. I would use an old 8x57 Mauser die cut down, or alternately, reamed out to handle the larger body/shoulder of the 8mm Siamese. You draw the temper out of a die by heating it cherry red, burying it in a pile of sand, and let it cool before machining. For limited use you don't even need to re-harden it.
I would use a .348 Winchester sizing die to neck down cases, and then necksize until it just chambers in the rifle. Seat bullets in a similarly modified .348 seating die. Use starting loads and fireform your cases. After that, you neck size only and reload.
The late George Nonte in "Cartridge Conversions" and "Handloading" decribes the use of home made dies when purchase is not feasible. I've done several obsolete calibers this way and it does work!
