Chris,
congrats on your new boat.I bought a74 tsunami 23 in 1997.Had the original 351 ford motor and mercruiser i/o.got 3 seasons out of it before the transom assembly fell apart.I repowered with a 2000 mercruiser 260 efi with alpha gen ll drive.The boat needed extensive glass work(new transom,engine stringers ,and deck repair) before the new motor went in,but it turned out great.I run her at 3200 rpms,cruises at 23-24 knots,burning about 7.5-8 gph.Very economical,quiet,all around solid power package.Feel free to e-mail me ifyou have any questions about your project at
bobfish21@aol.com. Good luck!
oh,and by the way,if you think your transom's ok,dont bet on it-at that age,theres sure to be water intrusion and very likely substantial rot. dont take a chance putting a brand new powerplant in a boat thats not structurally ready to handle it.