I am building a small copper heat exchanger I am going to plumb into my coolant system. I'm afraid that the copper will corrode my aluminum radiator or iron engine through galvanic action via the coolant. How was this prevented on older vehicles with iron engines and copper radiators? Or is this less of a problem then I expect it will be?
If the copper components are not in direct contact with the aluminum or iron parts, then there should be no electrolytic corrosion. This would require that non-conductive rubber hoses are used for coolant lines (avoid those with steel spring cores to prevent collapsing on the vacuum side, lest the rubber crack through and allow metal-to-metal contact). Also mount the copper parts on rubber shock mounts, rather than directly to the chassis.
Actually, corrosion was not well prevented in the past. For example, some Renault engines had an aluminum block with steel cylinder liners (shrunk with liquid nitrogen and dropped in place to expand for a very tight metal-to-metal seal, if I remember correctly). This short-circuited cell corroded rapidly. Modern antifreeze contains corrosion inhibitors such as silicate/phosphate mixtures that form an insulating film on metal and should help prevent any issue with the copper parts. Use a good quality ethylene glycol antifreeze with inhibitors.