Quick answers...

Yes you can increase boost, but to do so, you have to make sure you're getting that air in, and out, and keeping a good A/F ratio..

You can supercharge a non-supercharged Grand Prix, but's its expensive. You will need a whole new top end, Heads included.

Manual swaps have been done, and it will be expensive, and take lots of custom fab work.

That being said, I love the LSx cars, and LT1 cars, but the L67 in my GTP is my favorite to work on.