Woodguy - if you are talking about an Interarms Mark V, I think they are Mauser actions and should rebarrel fairly easily. The 243 has the same case head diameter as the 308/06, etc., and that may be close enough to the 6.5 Swede's case head size to allow the bolt to work. The 243 could also be called a 6x51/52mm and if the magazine is long enough to hold the 6.5x55 Swede, it should work.
I believe the Interarms actions are the same as those produced for the M48/48A Mausers and they chamber the 8x57mm round, so if it is the same action and magazine well it should handle the 6.5x55 Swede without a problem.
You may wish to run this question past the gunnut down in the Gunsmithing forum. I think he can clarify this for you. HTH. Mikey.