I ran a radiator mounted at the back of a couple of my Reliant trikes, but had the water running inside the frame tubes. I used a Fiesta rad' with the filler neck roughly level with the top of the thermostat housing, no header tank. Just used the standard Reliant mechanical water pump & an electric fan. It worked fine, but I think the distance travelled from the engine to the rad' & back again also helped with cooling.
Only niggle I had was filling & bleeding the system -I had to raise the back end on axle stands until it was the highest point in the system when filling & had a brake bleed nipple as an air bleed in the highest water hose.
On my red "Special Delivery" trike the bleed nipples were at the top of the frame front down tubes.
http://www.mankymonkeymotors.co.uk/Trikespageimages/potrike/trike.htmhttp://www.mankymonkeymotors.co.uk/Trikespageimages/greentrike/green.htm