Intake Leak
Well, you may have unwittingly found your problem.
I just walked aaalllll the way out to the shop (about 50 feet lmao) and here is what I did.
I opened my brand new Felpro gasket set for the engine and I put my untouched LD340 on my 360 SB with stock J heads.
I got 3/16ths of an inch without gaskets from bottom of intake to wall.
View attachment without gasket.jpg
And with a gasket I got 5/16ths of an inch from bottom of intake to wall. Kinda hard to see, the dang flash went off.
View attachment with gasket.jpg
I bet dollars to donuts it's bottoming out before it can compress and seal the gasket.