Sogilmore,
I’m in the same position as you. 73’ Tsunami with an I/O package, in need of a repower. The boat came with a Ford 302, with Merc II transmission and a Merc II TRS (31 years old). I’m not sure how the boat floats with this package since I’ve never had it in the water. I’ve just replaced the Transom with 2 sheets of ¾ plywood. FWIW, the wood weighed in at 90lbs before the install. It would break my heart to repower and find that water ends up on the deck! My best option would be to launch the boat, and then hump about 1,000 lbs of weight and put it into the engine box. It wouldn’t simulate the out drive weight perfectly because the drive would be further back, but would probably get the job done.
I see no reason for a large block motor in that boat. A 5.0 or 5.7 liter engine would be more than enough. I believe Alan from this site installed a Merc Horizon 300 Hp Bravo 3 in his Tsunami and got 50 mph. He hasn’t posted in a while, and is probably enjoying his “new” boat.
I’m very disappointed in Merc’s inability to come up with an innovative design for the stern drive. My 30 year old out drive design is no different than what they’re putting out the door today! They’re website also lacks technical information on there products. How about an installation drawing guys? I’m leaning toward Volvo’s Ocean series since they're the only company that has been able to think out side the box.
Marine Diesel's 6.5 liter engine package is within about 100 lbs of Merc’s 5.7 gas.
How about moving the engine forward to the gas tank area, using a jackshaft and saddle tanks in the stern? The baitwell is useless anyhow.