I have gone through this on my 70 K5 and my brothers 70 K20. Like they said the shroud is upside down. The flats with no holes go into clips that would be on the bottom of the core support. The radiator you have is correct for a 4 core.
I fortunately had a set of 4 core upper brackets that worked with my radiator tanks. For my brother they were on back order so I took his set and a set I had in my spare parts. Cut the two and welded them together to make the correct width. Then we cut the new rubber gaskets in half and glued them into the corners so it secures the corners of the radiator tanks.
I ended up going with electric fans after I installed an aluminum radiator and motor swap. I made a frame that uses the stock mounting points and he went with the stock fan shroud for his 350.
I have had to do the same thing for the radiator upper radiator mounts on my sisters 63 c10 also.
I attached a pic that has the lower clip in it. It’s from when I mocked up my winch plate.

Hers a pic of the top of the radiator. I have an LMC radiator cover plate bolted in between the upper mounts but it should give you an idea.

I fortunately had a set of 4 core upper brackets that worked with my radiator tanks. For my brother they were on back order so I took his set and a set I had in my spare parts. Cut the two and welded them together to make the correct width. Then we cut the new rubber gaskets in half and glued them into the corners so it secures the corners of the radiator tanks.
I ended up going with electric fans after I installed an aluminum radiator and motor swap. I made a frame that uses the stock mounting points and he went with the stock fan shroud for his 350.
I have had to do the same thing for the radiator upper radiator mounts on my sisters 63 c10 also.
I attached a pic that has the lower clip in it. It’s from when I mocked up my winch plate.
Hers a pic of the top of the radiator. I have an LMC radiator cover plate bolted in between the upper mounts but it should give you an idea.



