I spoke to Brian at IMM Engines about this a while back. Its due to the exhaust ports being further apart than on factory LA heads. He tried porting the header flanges on a customers' set of heads to eliminate the step and ran them against a set of TTI's. The TTIs still made more power. A little surprising but there it is.
IIRC he said he uses TTI or Felpro gaskets.
On a separate note my Schoenfeld 1 3/4" headers are pretty square at the flange so it doesn't hang in the port at all.
