With the bigger turbo it probably came with a HD actuator.
Adjust the rod and get the boost down to 13/15 psi.
Verify your fuel pressure goes 1to1 with boost.
Check the clamps(may have a broken one)try tightening the clamps for the hoses.
You may get lucky
If not the seal for the sending unit/pump could be bad or improperly installed.
That makes things easy and cheaper as you will have no way to make more than stock power.
Boost will be limited to around 15psi depending on how much timing you throw at it.
The 255 will be all you need for that.
Flush your rail and lines and flow injectors if it checks out use the stock stuff...
It would be a wise choice to send your heads out to be ported.
There are many porters out there that are reasonable and the irons can make alot of power on the right combo.
Comp turbo has a 45mm wastegate and a 60mm now.
You can pm me if your interested but...
In a nutshell
I would advise to get an external and put co2 on and tie it into your computer or electronic boost controller.
This will give you control over your boost curve and eliminate boost creep.
With a...
Converters respond to power period.
Our platforms will not go fast or have power on tap with bigger turbos and super tight converters.
These motors cant move the air at very low rpm.
A tcc clutch is a wear item no matter what its made out of.
We have worn out Kevlar clutches in wot situations on the dragstrip and street on high end converters.
Why many go non lock.
Ya kinda say right there.
5.0 1st
Now pulling rear gear would really need way below a 3.08
So the rear doesnt help the car at all.
This tranny is not for the buick world
But one can put anything in anything with enough money.
Your research should not of lead you to old cast wheel journal bearing turbos.
There are billet wheel turbos in bb even triple bb without hubs these days.
Also converter teck has come so far.
2800 rpm is not the right stall for the turbos you have chosen above.