Your talk is likely based on some theoretical assumptions about these engines (as you don't even know their size). I'm giving practical suggestions based on the knowledge about these engines.
Sure beautiful modern turbos are quicker, however all the plumbery you'll need to drive around this 90 degrees v6 will be quite long. And you could also use two small turbos. Some people place two turbos above the exhaust ports on these engines. Btw, the engine is odd-firing.
I won't comment about the fact of using a 375hp turbo to work under 270 (as you said).
I gave you issues of these (and not some theoretical) 24v heads: cost, weight, complexity. Compared to the 18v heads.
And these 24v heads don't flow better than the 18v ones, for several reasons. Perhaps slightly better.
All biturbo v6 engines are largely oversquared, the (supposedly torquey) 2.8 would be very happy at 8000rpm (the 2.8 is a 94x67mm engine). But you said you wanted the 2.0.
You won't need new sleeves because of the power but because you won't find an engine in a condition that's good enough to race. They are all worn because the last item was sold somewhere 20 years ago. Then you'll have to decide if you keep the stock 2.0 aluminum sleeves (so you'll need to restore the nigusil, or better replace with nikasil), or you go with the 2.5 cast iron sleeves (to rebore), but you'd switch to 2.5 (group A biturbo were 2.5). Making full custom sleeves is a quite expensive operation, but it's possible.
Then you can decide to pay a ghibli/qp4 2.0
24v block with its specific crank, or to use the 2.0/2.5/2.8 crank. The 2.0 24v ghibli/racing/qp4 crank comes with lighter and longer conrods (smaller big ends), the older conrods were pretty short for the 63mm stroke, and much heavier. All the v6 blocks have the same deck height.
The 18v/24v heads are inter-changable on the blocks, the late crank goes to earlier blocks (same main bearings). Avoid the latest ghibli GT blocks with different main bearings. But they are so rare that I doubt you'll find one for a decent price.