Water pump hose that runs to head?

I have a 200 inline 6 engine from a 1978 fairmont in my truck. There's a hose that runs from the water pump to the head, can I cap it off since I'm running without a thermostat? I know I'm not supposed to run without One but I have overheating issues Idk why my engine overheats but so far ive tooken out the thermostat, got a 3 core radiator, replaced hoses and now I'm wondering if it will overheat on the highway. Ive never tried it on the highway bc its a project truck and I'm still working on it. Once I solve this overheating issue then it will be good to go. I'm thinking it could be the fan since I bent the fins in so I could fit the radiator in but I'm not sure. I was thinking about putting an electric fan but I don't think it will fit since the water pump sticks out really far for the pulley. Idk any suggestions? I'm thinking capping off the hose from the water pump to the head Will decrease overheating so that water can just flow one way instead of One. Idk Im kinda doubting it but you never know. Please give me any suggestions thanks.

 

What's marked red is what I'm talking about capping off