I think initially I'd not view them as the same species. The lighter colored ones have a notably higher arching backbone and a bit different fin shape. I'd think they are not MGC. The darker ones you feature the most in your photos must be the MGC.
Have you made the thread on your Safari trip? I'd appreciate a link.
GMC is a very common abbreviation for General Motors Corp, which is why it appears most of us opted to abbreviate MGC Mekong Giant Catfish to avoid any confusion.
It'd be very nice if you attempted this. Very, very interesting (to learn what will happen at your expense haha...). Just beware that the posts of our peers from Thailand, China, Korea, and other eastern Asian cointries dealing with the MGC search on line and at local LFSs indicate that you might get any Pangasiidae under the sun, genuine or man-made, but not MGC. You will need to make 100% sure what fish they will be exporting to you or your importer and that this fish is most likely MGC.