I would like to use PVC pipe as a downcomer (internal riser) inside a 750BBL tank, to be used as an overflow to an adjacent tank. The product in the tank is produced water (with high cloride levels). Are there any issues with using PVC in this application? Would CPVC be better suited?

I don't think so. Unless heat is a factor, may as well stick to PVC. You might try leaving some in the solution over night or longer, and see if it affects it.

What kind of cloride, if its zinc cloride you may want to read this IngentaConnect Degradation of PVC: Effect of zinc chloride on the concentration ... Try Labman's experiment but leave the PVC in the solution for a week. The degradation is slow if it's zinc.