340 running hot

Nope, you want the coolant to flow faster, not slower [ post #41 ]. See link below.
Retarded ign timing &/or lean cruise mixture will cause hot running. W/pump. The gap between the impeller blades & the pump body should be minimal, almost touching. This ensures that the coolant captured by the blades has no escape route & that the coolant gets pumped into the engine.
Nothing pulls air like a 19+ inch, steep pitch, mech fan. Replace your clutch fan if old or questionable.

www.stewartcomponents.com