I made a restrictor with two 3/8" x 1/8" brass pipe bushings & a 1/8" brass pipe nipple. The ID of the pipe nipple is 5/16. Thats half the ID diameter of the heater hose. The bushings fit nicely in the heater hose. You just need a couple of hose clamps to secure them. I did this about ten years ago and the heater core has not failed. As far as heater output is concerned it is not affected. It still puts out plenty of heat when needed. The above-mentioned parts are available from any hardware store or home improvement store. My car is strictly a driver not a show car, so originality is not my concern. Hope this helps.
