You would have to measure the wheels to get the turbo size, its common for the covers to be bigger on the cold side, visually from the outside turbos look similar but that looks big i would guess older pte 6765 4" cover precision .63 housing
Reed at Work Turbo fixes older units like that
Once you put the brick wall in front of the radiator yes 160 is gone forever , but as the others said 200 isn't terrible , 215 is starting to get in the yellow zone.
Just state your goals with the car and everyone can help you with the easiest and cheapest way to do so.
we know pretty much what every combo will do before it is put together , think of us this forum as the Turbo Regal crystal ball office
You would have to measure the compressor wheel, the te44 and te60 share the same hot side T-31 turbine wheel, either or they are real close to the same turbo , 2mm difference and would spool the same with the Garret exhaust housing
There are many new turbo's out now that will work in the "stock combo" application , the old te44 wouldn't compare to a said pte gen3 5658 or an Hpt 5862 but your gonna spend, to utilize a 500hp combo you also need octane/fuel system/trans/ possibly a better converter then the 12" D5
I bet the throttle valve cable isn't set correctly on the 200
as for the th400 , it would be hard to kill a well built one in a 3500 pound car, and its half the price of a 2004r
Many dont agree with this but i run Vr1 20/50 in NY summer and 10/40 in the fall/winter for 20 years , with only showing a 3 psi hot idle difference
A front mount will put you over 200 degrees on the streets without good dual fans
Valve body, governor, speedo gear is different, its calibrated for a 305 and .373 gears, it can be made to work but wont handle much more power then a stock Turbo Buick has with out being built.