smallblock67

I have a C6 behind my 302 in my 67 coupe. The engine is a HO roller engine with an E303 so it's not a radical cam by any means. The C6 was built by a neighbor who is well versed in these trannies and works for a trans shop. He put a good shift kit in it and full rebuild. Now the issue: it shifts way too soft at high rpms but shifts very firm at lower rpm. I've tried a couple different modulators and have adjusted the mods every which way. I checked the vacuum of the engine and it is as follows: idle-15", high rpm under load-5",
No load at high rpm-22". What could be the issue? Thinking about just putting in manual valve body but don't want to spend the money right now. I'm puzzled and so is the guy who built it