I have 3b 23P props on my 342 baja. It is powered with twin 7.4 L MPI and Bravo Ones.

I am repowering with new 496 Mag HOs and Bravo one Xs. I am thinking moving to 28P props. Would that be about right?

Talk to Brett Anderson at bblades. Maybe he can help.

http://www.bblades.com/

Depends on a lot of factors. What RPMs where you turning the 7.4. How many RPMS do you plan on spinning the 496.
What was the increase in HP. It will take alot of increase to jump from 23 to 28

I would start with a 25p m+ or 24p b1

The 7.4L MPIs were turning about 4300 WOT and producing only about 305 HP.

The 496 mag HO produces 425 at the prop. WOT is close to 5000 RPM. Big difference in engines

If you were only turning 4300rpm try out the 23's that you have now on the new engine.

the 7.4 L MPI WOT is rated 4200-4600. The 496 mag HO is WOT 4600-5000. So really, I would think I would need to move up at least 3 inches anyway.

What year is the boat ?
Pre 01' boats usually have the drives mounted deeper,
where the 3 blades usually work best.

Most likely 26 Bravo 1 props. My neighbor has the same boat with 502/415's and thats what he runs.

Try the 23's first to see where you're at. You've got a 700 rpm window to make up as it is. If you're banging the limiter than it's time to move up. (You need to remember 1" up in pitch is at least 100 rpms less.)