If the lower neck is the OD same size as the water pump neck, it will work. The radiator I have (not Cold Case) has an angled lower neck on it, I just had to shorten my lower hose by 1/4" to take the slight kink out of it.
I've never seen any "factory" 273 rockers arms with jam nuts and my first 273 was acquired in 1966. Some one faced the top of the arms and added the jam nuts, Ed Hamburger used to sell them on exchange system I believe.
More information, please. What all was done in the building process, what is the casting date of the block, what heads are on it and what is the bore size?
That isn't oil, please look closely at the picture, it is a piece of metal, plastic or something. I see the oil in cylinders 5 & 7 and it looks nothing like what is on the # 1 piston.