Turbo 400 shifting problem

Hi, it's been a while since I've been here but a new problem brings me back. My son and I have just put a GM 350 with a BOP turbo 400 into a

1953 Buick! With the late model suspension and brakes it rides great and runs great.

Here's the problem. It seems to just want to stay in 2nd gear unless we shift it manually. Manually shifting her goes smooth as silk, 1-2-3, but she wants to start off in 2nd and stay there.

We have OK vacuum at the modulator and the line is clear. It's hooked to manifold vacuum at the carb base (quadrajet).

I'm stumped. It seems like if it were anything major it wouldn't shift manually either.

Any ideas?

Thanks in advance, Tim