The block is not the weak link. In order of failure basically stock pistons 1st. At around 500hp. They will last here but if you breath some knock on them at this point they will collapse the ring lands. I think you could get 100-200 more out of them at least for a little while. I have run 11-12lbs of boost with a supercharger for many years on these. Probably add a bit more from a turbo since parasitic loss is lower. Given a proper sized turbo a well put together motor with good exhaust system that can breath and head studs. I think you could get to around 800hp before the headgasket started failing. This is an assumption but I think a reasonable one. Cometic gaskets or something of that nature and some minor tweeks to the block and 1000+ is with in target. I made over 900 ft/lbs of torque with an om606 which is basically the same platform. You could make a lot less tq and make some good hp up top with the m104.
Stick with the smaller bore 3.2l and spend your money on a good tubular turbo manifold and a proper turbo that does not cost 50.00 and you can make a lot of power on these. Dump the stock ecu completely and go aftermarket. Way easier and more reliable. OE stuff is really old and you should basically replace the entire fuel supply system.