400b or 440rb

230 block is a heavier block with the cold weather option. Should only matter to you if you are building over 750 hp.
I say go with the 440 because it is complete, but don't assume it was rebuilt just because of a carb and manifold.
Check the dipstick and see if it looks like oil with a good color. smell it to make sure it does not smell like gas. if it is a chocolate milk color, walk away from it. Look at the bottom of the radiator cap to see if there is any oil residue. Look at the color of the coolant to see if it is rusty or a good antifreeze color. Those are things you should do when looking at any motor. If you can start it or not.
The 400 would be suspect because you do not know if it took water thru the manifold. The above tests may not reveal it unless it was significant. But you could pull the plugs and take a swipe to see if you come up with water. I took 8 plugs out of a small block chevy and was very impressed with the color of 7 ofthe plugs, until I got to a rear cylinder and got water pouring out.
Good luck.