Let us not forget that there are two different sized radiators for 1968.
1. 20 inch bolt in style, typically found on 6 cylinder cars and small block V-8's without air conditioning.
2. 24 inch clamp in radiators. This is the size of radiator found on big block Mustangs and Cougars and those cars with air conditioning. This radiator has both a much larger core surface area and larger capacity tanks. These are typically 3 cores.
If you are mearly looking to upgrade your cooling system, I would suggest the 24 inch radiator with stock installation brackets and fan shroud.
You didn't ask about fans, but the 4 blade style is adequate, but I like either the 5 blade flex fans or 7 blade clutch style fans.
Another school of thought for 4 core radiators is that the thickness of the additional row of tube(s) is detrimental to the overall efficiency of the radiator.