Twin turbo: 318 or 360, and why? This are your only options. GO!

OK, I vote for 2 turbo's, one on each exhaust. No need for 2 in series as that is just to get to higher levels of boost. Easy to get 15 psi with this size of engine and some common turbo's.

Mitsubishi 14G's will get 12-15 psi boost easily for 2.6L displacement..... more than you are gonna be able to use without getting better pistons, forged crank, etc. (Block will be pressed with 15 psi.... it'll be pushing past 500 HP if you intercool it!) The 14G Mitsubishi is easy to find, plenty of sources, and plenty of improved models for more boost and better low end spooling. 16GSuper with a better impeller would be impressive. Use the wastegates for sure. Will need some attention to keep things boosted evenly side-to-side and get the 2 wastegate responses equal; maybe there is some control to make that better. Surely that has all been done.

I'd for sure go want to go with an electronic FI for this; TBI would be adequate IMHO. But it will take some fanagling and patient tuning to make this work unless you can find some already made maps.

Not a project for the faint of heart....