Thinking about turbo on my/6 again...

Back in about 2005-2013, I used to mess around with the 1980s Turbo Chrysler FWD cars. 2.2 and 2.5 4 cylinder engines. When sizing a turbo, the main factor was the expected horsepower output. The tiny Mitsubishi turbos that came on the 2.5 turbo would spool fast, but they topped out around 200 horsepower due to exhaust flow restrictions. If a Garret t3 turbo was upgraded from the .48 a/r turbine housing to the .63, it would flow enough for around 300-320 hp.

With little more than a manual boost controller, my stock 2.5 / mitsubishi turbo could smoke the tires off a stoplight, often without trying.

I'd think that whatever size Garret turbo came on a factory Buick Grand National V6 would be more or less right for a stock / mildly modified Slant 6.