The 40 mph top end will cost you up to an extra 4G over a 37-38 mph top end. The Tsunami/Sceptre equipped with the OEM Mercruiser 5.7L 260 hp turning a Alpha drive could achieve 37-38 mph. To get to 40 mph, I figure you need 300-320 hp, assuming the Alpha I/Alpha Gen II drive.
You might achieve 40 mph with 260-270 hp and a Bravo III, or Volvo 290DP drive, because of the increased efficiency of the counter rotating propellers.
A quick look turned up this:
Sterndrives.com (not a recommendation)
5.0L 260 hp with Bravo III complete (no shipping or installation) $11,525
350 Mag MPI (300 hp) with Bravo III complete $14,331
MX 6.2 MPI Horizon (320 hp) with Bravo III complete $17,056
Figure $500-750 for shipping, another $1500 for installation, $1800 for the FW cooling package, and throw in $2000 for incidentals like gauges, monitoring systems, and other unexpected stuff. And this assumes your boat is ready for install.
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Common Sense is learning from your mistakes. Wisdom is learning from the other guy's mistakes.
Fr. Frank says:
Jesus liked fishing, too. He even walked on water to get to the boat!
Currently without a SeaCraft 
(2) Pompano 12' fishing kayaks
'73 Cobia 18' prototype "Casting Skiff", 70hp Mercury
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