First, welcome to the site. Not trying to "rain on your parade", but just rotating the cocking tube 180 degrees wouldn't help. Rotating about 60 degrees would work better, I would think. You'll also have to put the notch to hold the bolt back in the other direction or you'll have to go up instead of down with the "HK slap" to close the bolt with your right hand. Is your Uncle going to machine you a new left hand bolt and are you going to move the ejector lever from the left hand side to the right hand side of the trigger frame? And what about the ejection port in the receiver?
It seems to me that there is a lot to be done to make a roller lock for a "lefty". I have seen a couple of right hand cocking tube projects over the years here. Have you shot a G3K left handed? Just reworking the cocking tube assembly won't do anything about the .308 brass in your face. Again, I'm not trying to be a "wet blanket", I just asking how far you plan to go on this project? Maybe you have shot a G3K left handed and like it. I don't really know. But boy wouldn't it suck to put in all that work into a right hand cocking tube assembly and then you can't stand the .308 brass in your face. Making a left ejecting G3K would be a huge project. Can't say as I have seen that done.
I'm just bringing up a couple of things that occurred to me. Maybe you have it all covered. I've seen some cool custom pieces here. I'm fortunate to own a couple of oddball things, like a reverse stretch in .45 ACP or a .40 S&W SD with welded rails instead of the rubber hand guard. But I can't say as I have seen a left ejecting roller locked. I've seen a couple of left handed shooters use full size 9mm guns, but I can't say as I have seen anyone shoot a .308 left handed. I'd think it has been done. I just haven't been around someone that does. The right hand cocking tube assembly modifications I have seen have all been on 9mm guns. At least that is what I remember. A home built roller locked is quite a project. A left hand home built roller locked sounds like a very advanced project. I sincerely wish you luck with your project.