Hello

Have a 1980 Datsun 720 pickup I need of an exhaust manifold. Is there something after market I can buy or some way to understand if my exhaust manifold needs replacement? It's very loud going down hill without accelerator pedal applied. This of course is the result of an overheated engine from which a broken fan belt had caused. machined head works good and starts up like a champ. It is just loud going downhill. A double gasket didn't take. I've been looking for a part for about 18 months now as a replacement. Even searched Mexico which they have a different version of this exhaust manifold. The intake and exhaust are combined on this model. Can someone give me some ideas how to overcome this problem preferably with smog restrictions in mind? But not necessary. One of my friends had mentioned to use a straight ruler to determine if it's warped or not. And another cautioned me that it could be a fine crack that is undetectable by eyesight. If it is slightly warped it can be filed down as a cheap fix. I'm looking for good ideas that would help me decide which path to take before I start disassembly. Anything would be appreciated. I'm a former owner of a 5-10 Datsun 72 but have come to love this 720 8 foot bed truck. Any suggestions from you Datsun aficionados would be great.