When I got my 85 pickup the manifolds were off the truck, and the firewall was coal-black. That gasket had blown out on it, and the POs son had pulled it apart to fix. It had sat apart for a couple of years like that. The old buzzard had said that he thought he had scrapped the manifolds, his son was there when I loaded it on the trailer and dug out a set of manifolds that the old man either forgot about or didn't know was there in the first place.
Some idiot had poked a 1" hole in the wall of the exhaust manifold where the inner bolt closer to the head is, and ruined it. I found one in a local junkyard and had to take it to a friend of my son's, who has a machine shop to drill out and retap the 2 outer holes for me, and then when I got it on the truck, it would barely run for many reasons/ the worst of which was that the front cat had disintegrated, and it's guts had clogged the rear cat. This was what blew out that shim steel gasket originally in the first place. I went dumpster diving at a muffler shop and got some nice clean stainless steel bends off of some sort of newer vehicle, and cut the original front cat off, used that flange and pipe stub, welded up myself a new exhaust from the manifold to the existing muffler. Only after doing that, and straightening out the old buzzards wiring hacks I finally got it to run.
You might check for some sort of exhaust restriction retaining heat, as to why yours blew out.
A funny continuation to the story of my truck, I was getting gas in my daily driver Dakota one day after work and a 91-2-3 D150 pulled up on the other side of the pumps from me.(unsure of exact year, it had the ugly grille was all I knew)
I said something to him about wishing I could find a decent older truck like that and somewhere in the conversation the town 2 over from where we were came up, he said he knew of a couple of them over there.
I told him I bought a maroon and white one over there, he asked if that was the one with the slant 6 in it, I said "yeah".
His reply was that "oh I know that one, it has no compression" I laughed and told him what was really wrong with it, I haven't measured it, but once I got it uncorked it runs fine. I didn't have to do anything to it that has anything to do with compression issues, I have not yet had the valve cover off of it. The stare that he gave me was one of those "if looks could kill" moments.
