Is the radiator stock?
Does the overflow tube come out of the cap area or the tank?
Does the fan spin the correct direction? (serpentine belts use reverse rotation fans and water pumps)
As mentioned above, you need to know the limits of the radiator and heater core, the old stock ones may take up to 5 to 7 psi caps or they could be zero pressure units, new modern radiators take 15-17 psi caps. Trying a high pressure cap on an old radiator will likely blow out heater core or the tanks on the radiator.
Try moving the fan closer to radiator, spacers are fairly cheap and you will get better results with a mechanical fan. A shroud would help, but not if the fan is to far from the core, and if you get it close enough you may not need the shroud. If you go electric, buy the best you can afford with the most cfm available. You will need a decent alternator to power the fan at low speeds as they pull a lot of amps. Use heavy, 14 gauge wire with 30-50 amp relay, and a temp control to turn the fan on/off as needed. Manual on/off is a sure way to run your battery down when you forget to turn it off!
Be sure you have 50/50 coolant, or straight water if you are in a hot climate. Flush the system to be sure it clean just for added insurance.
Slow speed over heating is a sure sign of lack of air flow through the core.
