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I’m gonna go single I/O from the twins. Thinking 454 or larger. But would a fuel injected 350 or 383 work? Or the torque from the big block needed?

That's is a good question. The IO is more efficient than inboards, and I know there have been 25 conversions to inboard. Hopefully someone with a 25 will help you out.

I have a B25 Bahia Mar with a single DP volvo 8.1L (496) @ 375hp, and a B25 sport convertible with a single DP mercury 6.2L (383) @ 350hp.
Very similar performance characteristics.
Both lope along very efficiently @ 25-35 mph and top out in the low to mid 50's.
Performance doesn't change much if there's 4 of us in the boat off to a dinner out to a restaurant, or 8 + all the gear headed to an afternoon picnic.
Probably because of the DP outdrive.
Depending on your performance expectations, you might get by with 300hp(?).
If I did another B25 repower, I'd be leaning toward another 6.2L in that 350hp - 430hp range.

Thanks for the numbers John!

My mind has gotten soft from .... well a number of things but the group of canooks up on Georgian Bay have done a number of single replacements on 25’s. Ian Upton comes to mind as well as Doug McKensey. Single big blocks and such. A lot of this was on the old 25 site which has gone the way of the carrier pigeon but you may be able to pm them for some input. Very helpful good guys.

A single small block 350 propped right has plenty of power. There is a member with a single 150hp 4 cylinder Mercury Four Stroke and gets 30mph out of his 25ft Express.

I have a 383 mercruiser single inboard , tons of low end torque for a 25. Gets on top quick.

my buddy has a 25' fibfab and went with merc 383, he had a bravo 2 drive which seemed to be quite expensive.

what are the folks running for outdrives on the big single?

They are about half as much as twins...

I don't think you would get away with anything less than a Bravo or a Volvo SX or Duoprop.