456 gear with 29.5 slicks!

On my 68 340 barracuda I run 4.56 but I have only run it with MT ET streets 28 inch. They are actually closer to 27. They were perfect with my set up. Then I went with 28 inch MT slicks. That killed some, But were ok. On my 340 Duster I have 4.88 and 29 inch slicks. I go through trap with them at 6800. My barracuda trapped at about 6200 with the ET streets.