Low restriction is the name of the game. When I put a 2.25" reproduction exhaust on my GTS I was in a hurry and used the supplied clamps. First good flat footed gear change blew the mufflers off of the intermediate pipes. Keep in mind this was the stock 383 with manifolds, etc. That really got me thinking I needed cut outs.

After the 451 went in I added the cutouts and did some dyno testing. We measured a consistent 25whp gain with no other changes.

Regardless of the tube size, cross pver type etc. your problem will persist until you have a higher flowing set of mufflers. Just my thoughts from some testing we did. Interesting subject!