I don't think the length of the bolthead will matter, since the "difference" will be taken up when you screw in the barrel and headspace it- but I'm not sure.
My thinking is you're just going from a magnum bolthead to a .308 one...
Suggest you call PTG- they've got some really great precision products.
http://www.pacifictoolandgauge.com/s...erformance.htm
Got my Savage factory barrel off last night (wow- what a biatch) and screwing on the new one today, I also am replacing the factory recoil lug with a precision lug that's machined, not stamped like the factory lugs. Worth the extra $30 while you're at it..