The bigger 4 series turbo changed the whole dynamic.
You have fallen victim to the Law of More, as in if a little bit is good, More bigger must be better.
The new turbo is too big, which is why you had to rev the engine so high to make boost. Chevy did this intentionally with the Corvair, so it self limited the boost to 5-7 pounds without a waste gate, but this also made for a lot of turbo lag. Newer designs used smaller turbos which spooled up much quicker, and then limited the boost with the waste gate. Also, your exhaust manifold has a lot of volume; you want the hot exhaust gases to expand in the turbo, not the manifold. Look at the typical turbo header, which is a simple log manifold.
You may also be fighting a case of Bigcarbitis. When the carb is too big, there isn't enough air flowing through the carb to pull fuel up from the bowl at low engine speeds, so the jetting must be increased to compensate, then at higher revs when the airflow increases the mix goes rich. The typical scenario was the guy that put an 850CFM Holley on his stock 283 Chevy. You could make it work, but it never worked well over a range of RPM and load.
For comparison, the Chevy turbo made 180 HP from 164 cubic inches using a 1 3/8" (35mm) carb, that was originally used on the first Corvettes with the straight six.
My two cents....
