When you are doing such a project, you really need to take off your exhaust manifold on the car & actually install your 2.5" exhaust manifold to see where everything is going to be laid out. (less cutting & re-do work IMO.)
IIRC, Tlowe said the the 2.5" manifold exhaust hole is positioned a lot more fwd as compared to the stock 250 manifold & would most likely sit directly over the crossmember/frame? Saying that (& I am only quessing you would need a really tight radiused pipe. Just quessing here because I have not seen your set-up first hand.
Not sure the thickness of your square flange (3 hole) you made? but, Might I suggest using a thicker metal flange.

I would suggest a thickness of .375" or 3/8? And it should be chamfered like the exhaust manifold is to kinda centers the donut gasket? I am thinking your plate is going to distort.
Just a suggestion.

Definately reshape the round section of tubing for the T4 flange.

When you are welding pieces together, make sure there is no extra slag/weld inside the tubing that can come loose (dislodge into the turbine wheel & distroy it) & or restrict airflow.

Looks like you are using a stick/arc welder?

BTW, just some FYI, on another forum a guy w/a supercharged Cadi STS-V 4.4 liter (bigger than your 250) switched over to a turbo set-up, he was running a .81 A/R on the turbine housing & his turbo would not get full boost until 4000+ rpm.
Guys on that forum were telling him that the turbine A/R was fine for what his combo was, I suggested to get a smaller A/R for the turbine & surprizingly \:o the guy took my advise against all the others advise & he was happy he did, by switching to a smaller .68 A/R turbine housing(IIRC?)the boost comes in much sooner.

He also had his torque converter restalled to 3000 RPM & got reinstalled & now it stalls to 2800 RPM, it actually needs a few hundred more RPM stall to get rid of the turbolag.

If he would have stayed w/the .81 A/R turbine housing the engine would have needed approx 3600- 4000 RPM stall.

I advised him he would not like to wait for the boost to come in so late in the RPM & running a 4000 stall converter on the street would not be a happy thing to deal w/on a street car.

He is still doing more tuning but when he dynoed it made 502 HP @ the wheels @ 13 psi of boost pressure.

Just trying to reasure you about the A/R choice on your turbo, going to an A/R of .84 or bigger, you definately would not like it for a daily driver. That's all. ;\)

I would also like to add that I am not bagging on your work @ all, just want to see you succede.

BTW, is your dad helping you @ all w/this project?
Did he drag race his V-8 sled,,um I mean Buick @ a track yet LOL. ;\)

MBHD

12 port SDS EFI