340 idles at 1500 won't stay running in gear

72 dodge demon 340 with a 408 stroker and a 727. Here's some back story. i was coming from a cruise and immediately noticed it running rough. as i came to a light i noticed the headlights dimmed, it started idling rough and eventually died. It started right back up, put it in neutral and played with the gas to keep it running. I had to keep doing this all the way home, slap it in neutral give it some gas.
Got home checked the battery is was 12.87v, checked the alternator it was putting out 14v at the battery Picked up a vacuum tester, in idle I'm pulling 10 inches and in gear it drops to 5. I do have a pretty aggressive cam but don't have the specifics, i believe it has a solid lifter since it was an older bracket racer running nitrous .
I've cleaned the carb, pulled the front fuel bowl and checked the jets. Switched the plugs and they where fouled from not running it alot, dry carbon couple where a little wet.
Right now it will start up and idle but i have to feather it up to about 1500 and it will stay at that with my foot off pedal running. If i put it in gear it dies almost immediately.
I was able to get it to idle then in gear and drive a little down my street but then i could feel it start dying. This was a couple days after it initially went down and it has progressively gotten worse. I've also noticed that i have like a black soot built up on the outside of the manifold on the drivers side down by the valve cover.

the motor was built in 08 by koffels place, didn't get any build specs just a few things i was told when i was talking to the previous owner over the phone while driving. It does have a holley street avenger 870 with electric choke, and a edelbrock rpm intake manifold edelbrock heads.

I think it's a bad intake manifold gasket, tried the spray test but didn't notice any thing. I've also checked the vac line to the pcv and at one point blocked off all of the vac ports on the carb .
I noticed the current intake manifold gasket is like a blue paper 2 piece gasket.
I'm thinking intake manifold gasket, if it sounds like it's bad what are some tips/ tricks for replacing it, I've never done one before