Intake Manifold Gasket Change ?'s

[94Reborn]

I got my truck finally running well last week after my engine swap. I was on a cruise for 5 days and just came back today to a surprise. When I started my truck it sounds like its missing, the exhaust is almost pulsating, and the engine is a shaking mess. It sounds like the valves are all tapping loudly as well. My thoughts are that the upper or lower intake manifold gave it up and I have a bad vacuum leak. A spray with some starting fluid around the intake plenum seemed to have a small effect, making it run even shakier and rougher when spraying near the FPR area on the passenger front side of the manifold.

It sounds like the upper manifold gasket is simple, just 6 bolts and lifting it off. To do the lower, I need to do the valve covers and there's a risk that I could cause coolant leaks if not resealed properly. Should I try just the upper and hope for the best or is the entire lower job not really as bad as it's made out to be? I'd hate to have to waste an upper manifold gasket if it doesn't fix it the first time. But which gasket is more likely to fail?

Also, maybe its just me worrying, but the oil level seems higher than it was when I changed it 2 weeks ago as well, and smells a bit like gas. Is gas contamination in the oil a symptom of a bad upper or lower manifold gasket?

 

[94Reborn]

Update, I changed the upper manifold gasket. The idiot that had the engine before me left a piece of the old gasket stuck to the manifold and it was breaking the seal. Cleaned it up and replaced the gasket and now the leak is gone but that was not the rough running problem.