And when you say its "blowing through" the converter for about 150 feet, what do you mean exactly? To me it sounds like the car is happy with the 8" converter, it just may not be something you were used to with the 9" J converter.

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When I launch the car now, the RPM's are going to about 5000 or so, but the car isn't really moving. Kinda like the clutch is slipping in a manual transmission car for 150' before it starts to hook up. After about 150', it starts to get hooked up and then the RPM's go up enough to shift (about 6000 rpm's)
The car was fairly happy with the 8" converter before I changed the heads, intake and camshaft. Not so much now.

Brian