kb,
I would say that is true in regards to 30" shafts on less than 225 HP. I'm sure you can do it (just swap out the LU), but most likely at a cost premium. Your 250 is set up perfectly for twin 150s, 25" shafts, that is what I would be looking at.
Obviously, you've thought about Etecs as your rigging mates right up, unless your Rudes are so old they use the engine harness with the big red plug at the engine end. Even the twin binnacle will mate up, using type 479 OMC cables (they haven't changed over the years). You change over brands, and then you start to deal with an extra $2K or so in rigging, i.e. harnessing, binnacle, gauges, etc. A pair of Etec 200 H.O.s would be sweet as well...