Your '57 235 would have been born with a 1.5" outlet at the tstat housing and a 1.75" inlet at the water pump. The short water pump sounds like it has a 1.5" inlet.
Your 1.25" upper hose is incorrect...you'll need a 1.5".
You can buy a reducer/expander at most FLAPS to solve your 1.75" to 1.5" dilemma.
Your question about bottle neck is valid, but you will encounter the same problem regarding flow if you switch radiators to one with a 1.5" outlet. The reducer/expander isn't elegant, but it will take care of your issue.
Matt
