I'd do them the same as a tenderloin, on the grill. The last time I tried something like that I didn't like the texture of the meat since it sat in a marinade for so long.
I have done them on the kettle. Marinated in a ziplock bag with a bottle of Wishbone Italian Dressing,
indirect with some applewood for smoke, at about 300F, then finished direct for a bit more color.
I have had the same result. Too salty to even eat in my opinion. I just get the pain ones and season myself and always grill and not on the WSM but i do throw some apple chunks on the charcoal to get a little more smoky flavor.
Do the loins have a brand name or are they store-packaged? I've seen Hormel and "Cherrysomething" in the stores and IIRC it was the latter that had the salt issue.
I've done a lot of those. The wife buys them by the grocery cart-ful every time they go on sale. I strongly recommend not trying a low and slow smoke on them. Whenever I've tried to smoke them they've come out kind of mushy. I think it's because they're in that marinade so long. Best way I've had them is a hard sear on all sides and then let them finish off the heat for a few minutes. Leftovers make fantastic sandwiches the next day.