The m38 Turks are virtually all going to be threaded for the small ring barrels (though they can take any of the high pressure rounds that a regular 98 can). If buying pre-made barrels then MidwayUSA has Adams & Bennet small-ring barrels in 6.5x55, 7x57, and .257 Roberts, all of which should work fine (and if you have a C&R then with dealer discount those barrels are only $87). Many of the custom barrel makers will be cutting the threads as you order them anyways so if you order from them it's not a problem.
The m48 takes a standard thread size barrel, but the receiver is slightly shorter. As such many of the parts don't interchange with a standard 98.
If you don't plan on rebarreling though, then you'll just drill/tap and have the bolt handle forged. Make sure that you can your own bolt forged rather than buying a new one or you will likely run into headspace issues.