Regardless of the type of baseboard units you use, they will be considerably longer than the radiators that they replace, so there is no way to premake a “retro fit” product. If you mix cast iron radiators and copper baseboard, the copper will heat up and cool off faster than the higher mass cast iron radiators so some rooms may get warm faster and also cool off sooner, or others will heat up slower, but then overheat, depending on where the thermostat is installed. Cast iron baseboard will minimize this, but you will have to use more baseboard than with copper.