I would build it on the long action. Yes the chamber reamer will take the 6.5 Swede to 6.5-284. I see the rim diameter on that cartridge is listed at .480 same as the Swedish whereas the parent .284 is listed at .473. The bolt face probably won't need any work. The specs show the 6.5-284 has a .002 smaller OD on the neck, so that portion of the reamer will not touch except for the last few thousandths. I don't know what issues you'd run into getting the .284 to feed from most actions, auctually don't think it would be that bad. If you went Savage they make a rifle in that chambering so it would be as simple as buying their magazine box.
Still Learnin' as I go!
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