Don't get frustrated you ask techie questions. Seems some have built close to 275 maybe almost 300 hp at crank 6.5 drivable and working trucks.
Getting 250 + at wheels is the next step up and few do it mostly a little at a time. After the 250hp at crank mark money spent and hp developed is like hot boats and race cars the cost goes up exponentially with hp/speed increase. And you have to start making some trade offs low end loss and drivability etc. Again few do it.

I am no expert but have read like John says around 80 mm^3 (81-84?) max for a DS pump. I think Westers garage modifies a DS pump for a bit more (maybe thats the 91 you posted). I think they configure a different cam ring or something internal. Read Pennsilar turns up a DB pump for thier hot rod 6.5's. One guy even mounted (2) DB pumps on a tractor for more fuel. I have read Heath's legendary 500hp 6.2 used a different IP entirely something off a medium duty truck.

Just some suggestions for you to get the last 5-15 hp possible out of a DS IP 6.5. Timing gears, feeding the beast mod, FluidDamper balancer.

Is this going to be a race motor, street truck, sleeper, or work truck?