i've gone thru 3 exhaust manifolds on my '65 sub.w/292 in the almost 40 years i've owned it. I found the problem to be they warp in the back, of course where its hard to see. my truck usually runs cool and i was surprised to find the manifolds had warped. the first two just warped over a period of 10-12 years, the third happened quicker but i was using the original radiator which had a few pinhole leaks, in the arizona desert in summer. if you pull the manifolds you probably will find some evidence of exhasut, like soot or discoloring. torquing the manifolds in the proper sequence to proper torque as listed by chevrolet helps if the manifold is not too warped, double gasketing helped for awhile but bolts seem to work loose fairly quick ,requiring constant retorqueing . i got repalcements at the junkyard from pickups.
