Wow, a month late on this. Hope it isn't completely useless.
I think you just need a good 318 or Magnum gasket.
My understanding is the mismatch between LA and Magnum ports only comes into play when using 340/360 manifolds (and headers?) on a head with reduced material under the port (EQ or MP R/T, for example). Best I could tell, the Magnum port is a match to the 271/318 port, and on the OEM heads a buddy of mine had, the 340 manifolds I have would have been fine as the sealing face went all the way down to the deck. But on my MP R/T heads, the head is undercut below the port leaving not enough material to seal the manifold.
Because of this I had to build adapter plates that had the port from the Magnum on the inside and the outside matched the outside of the manifold. These started leaking and then leaked immediately the second time I used them (although that could have been installer error, too).
I have planned to go to a smaller port header someday to get rid of the adapters, but haven't gotten that far.
Either way, I kind of doubt the issue you have is due to a port mismatch.
