I have removed the oil cooler from my LT1. I intended to simply connect a hose from the radiator to th water pump in place of where the cooler was, but the radiator side is 5/8" and the water pump side is closer to 3/4" or so.

Where can I get a step-up fitting or does someone have another solution to this problem?

 

You just by the bypass hose that every other f-body LT1 uses without the oil cooler.

 

You can also cap off those connections; no need to connect them. I ran my car that way for 4 years with no change to the cooling system.

 

Is this bypass hose a GM part or can I get it at any parts house