Worth trying with oil cap off just in case. Little effort required. One test I hope somebody out there can comment on. If exhaust is that restricted my though is there will be almost nill intake suction or certainly sub standard. Perhaps a piece of paper held horizontal a little higher over the intake port inside the air cleaner might indicate if suction is the same as the good comparison car. Lucky you have a comparison car. Even when cranking I suspect the troubled car will develop little suction in comparison to the good one if the exhaust is pretty well plugged I would compare suction cranking with my carb syncronizer because i have it. All you want is a comparison does not have to be scientific. Was thinking a flat plate with a fitting to accept a vaccum gauge might work for a comparison test. Really though reading the back pressure on the exhaust side seems the route to sanity. Again since you have a car for comparison you do not even need a manual. Unfortunatly the bolts in the test hole on the exhaust manifold may be pretty tight if not seized solid. Also liked the ideal of loosening the exaust flange a little to see if any improvement. Since the engine reciently has had major work at least the bolts will not be seized for that. This kind of thing can be pretty frustating. Just random thoughts but might get someone out there thinking about a positive test. There was an english guy a few weeks ago with a much newer model that I suggested he drop his header pipe for the test. Yours sounds worse at least his would still run but with very little power. It proved it was his converter that was plugged.