The 250 L-6 is probably one of the BEST, smoothest running engines ever installed in a boat. "Long-Blocking" [rebuilding] one shouldn't cost all that much but if I'm not mistaken, the manifold and riser are getting more expensive these days..........
The 4.3L V-6 would require side engine mounts (read: a little fiberglass work) PLUS, maybe your engine compartment is a tad narrow [read: a little more fiberglass work]..........
You would also need a slightly different drive ratio, since the WOT rated RPM for the 4.3 is a bit higher than a 250 L-6. (maybe a little more than a "bit" higher)
If you could find a rotten boat with a COMPLETE newer (Alpha Gen II ) 4.3L engine + drive, it would be absolutely worth the swap. But understand you would need to do some engine compartment "stuff" to make it all fit properly AND it would be more money over just rebuilding your 250 and bolting it back in...........depending of course, on how cheaply you could buy the "donor" boat.
It would be a worthwhile project if you wanted to do a winter project. If you have to pay someone to do it, you might just be better off just getting the 250 rebuilt and bolted back in. (Don't forget to replace the manifold, riser and coupler while you have the engine out.)
Cheers,
Rick