Best Way to Plumb heat to 24 x 26 apt.????

I have a Profab 400 Outdoor Boiler (which I love). I want to bring a supply and return line into a 26 x 24 space above my garage. It is well insulated and new construction.
What is the best way to plumb in the heat. I want to use a couple old restored cast iron rads that i like for heating purposes. I have thought about a small HX with a circ pump (Grudfos Alpha) that will run 24-7. Run from boiler is about 50 ft. Can some one explain the best way to hook up the rads to the other side of the heat exchanger with a thermostat. OR - is there a better way to do this?????? Thanks