Great choice twh731!!
I built a very similar gun a couple years back.
Savage SA with a Shilen chrome-moly match grade barrel with an S7 contour (savage varmint contour), although I went with a McMillan F-Class stock.
Oh...and it's not backwards either ;)
I haven't shot any pills that large yet...been fireforming with 123gr Sierra MK's and a stout load of H1000.
Any info I found on 139 Scenars was with 47gr of H4831SC, obviously I'd back off and work up, unless you're fireforming, then I'd leave it at a full load to help with getting your cases formed nicely the first time!
I didn't need to form the false shoulder, just screwed the barrel on with a 308 go gauge and I have a nice slight crush fit with a factory 260rem casing.
I've been firforming my loads out to 780 yards and the best group so far was a couple weeks ago with an 8" group...while fireforming!!
I can't wait to actually dial in a load or two with the 130gr hybrid Bergers I have as well as the 120gr A-max!!