Doh! I have to take my foot out of my mouth. The 77-81 radiator has a pressure cap, the 82-85 radiator has the cap on the reservoir. Still, reservoirs abound at the PnP.

Since your original radiator has brass end tanks or whatever end tanks of radiators with metal end tanks are made of, can't you take it to a radiator shop to be repaired or recored?

Sixto
87 300D