TH400 issues

[Mrturner1]

Ok so I have a 1970 c10, mild build small block with a th400. So with the gear selector in D (drive) when I'm in wide-open throttle from a dead stop it shifts from 1st to 3rd when it should be hitting 2nd. But if I put the gear selector in second from a dead stop and floor it, it shifts into second just fine and barks the tires, and holds 2nd til I shift into 3rd.

Any ideas?

I was about to bring the truck to a tranny shop to get my much needed stall converter (which I'm so excited about, thanks you guys for steering me that direction) and told the guy over the phone exactly what the tranny is doing as explained above, and he says I'm lacking vacuum because of the big cam, and need a vacuum boost canister. I know he's not just trying to sell one because he doesn't even carry them or install them.

He quoted me 525$ for a 10" race converter built local with billet parts and is comparable to high end Coan converters. And another 500 to remove my tranny, inspect, and re-install with new converter.

I'm not a tranny guy, but I do know th400 is an awesome tranny and strong as heck. Can anybody offer some insight and ideas for what to do next? For anyone reading my head and cam combo thread, I decided to take your word for it and leave the motor alone and focus on these obvious issues, so this is where I'm at