It is obvious that you don't need a new rifle/caliber for shooting SC deer at 200 yards or shooting gongs at 100 to 600 yards.
agree
From the sounds of it, you just want to build a new rifle in .280 caliber. Bingo! I would suggest just building it from parts as your finances allow, pick up an action when you see one for the right price in the classifieds or else where, as well as finding a stock the same way. Don't destroy or take apart any of the classic caliber model 700 Remington rifles you already have. The .270, .30-06 and the 7RM are all classics with readily obtained over the counter ammunition if necessary.
Thats pretty sound advice
To me it is obvious that you need to build a 4th rifle ... of course, just can't live without it...!! and build it from parts other then your current rifles. You don't need a .280 immediately such as for your SC deer hunt, as any of the other rifles will fill the niches you want for the short term. But you want a new .280 so take the time, buy parts as they become available and get her done in 6 months to a year as finances and time permit.