its not that a larger turbo will just flow more air, you should really look into turbo sizing when talking about this. A T25 at 8psi only flows so much, a T3 at 8psi will flow a lot more, yet at the same boost pressure. On a 4.3l he will have much more air displaced from the larger amount of exhaust gas making it a lot easier to spool a little bit larger turbo like that.

high compression? Not really, they run around a 9:1 ratio, thats not high at all for a low boost setup and cast pistons are perfectly fine for that.

please please please read up more on turbo sizing before recommending something like that again.

no offense to your knowledge, but what you had written above wasn't really correct...