I have, (had) a raypack 336a that was living on borrowed time. A polymer header started leaking recently at its junction to the metal heat exchanger. The Headers were fine but the bolts had rusted off. I thought I could replace the bolts, but the inside of the was so rusted that the heat exchanger was the only solid piece. A new heater 336a is on its way. I just wish raypak would use stainless on the interior parts.
Rather than jury rig PVC to keep the pumps running I took off both polymer headers, puled out the flow diverters, and bolted them together with their gaskets. I left in the thermometer, and pressure switch to keep the holes plugged. I plugged the relief valve with a pool stopper. twelve 1/4x20 x 1-1/4 bolts and 12 washers for $5.30. The old headers are now a quick and easy bypass until the new heater arrives and fits perfectly.
Hope this helps someone else. I will keep the old header construct for future heater repairs.