Actually I understand they built them on the Long Lee the predecessor to the Smelly. The Long Lee action doesn't have a charger guide.

I don't know what cartridge that is you have next to that 303 but that isn't a.375 2 1/2 NE. Here is what a .375 2 1/2 NE case looks like next to a .303.

I shot up all the loaded ammo I have in .375 NE yesterday or I would have put up a loaded round.

And yes I have thought about building one on a Smelly. I think that the Charger guide can be removed fairly simply. The stocking and barreling is straight forward.

The only tricky area that I see is the magazine. The 375 2 1/2 NE is just slightly longer than a 303. A factory loaded 375 NE won't fit the magazine because the round is to long. The front magazine lips and guide ribs would have to be adjusted or the magazine converted to single stack. The cartridge would then need to be loaded to a slightly shorter OAL.