Smaller size tubing to keep the flow velocity higher and put a Lincoln electric fan on the radiator

splice in a section of clear tubing/hose to see if you have air in the coolant flow(foam) or solid water flow.

you definitely have built in a lot of places for air/steam pockets to form and WILL continue to have problems until you get all the basics about liquid/air flow corrected

I might suggest that if you want to continue with the rear rad that for longer trips you mount a pump driven off the driveline(trans or rear end) to augment the flow