please consider a mercury repower drop in.
It will literally take your mechanic a day to drop it in, and EVERYTHING will be warrantied and more importantly warrantied at ANY mercruiser dealer.
They come in such a way that the only thing you use from the old set up is the steering and shift/throttle cables, everything else comes new.
On a Friday afternoon when you need something warrantied the fact that some guy on the other side of the country sold you a motor wont do you any good to get your boat in the water for the weekend.
When I checked 2 years ago 5.7 engine, drive, transom plate, trim pump, wiring harness, etc... depending on drive choice and engine options, will run about $12-$14k installed.
The labor is key- because the swap is so easy the labor should be less then messing with long blocks or trying to re-use old stuff.
We went with a 4.3 fuel injected motor because the boat was smaller and we wanted a little less weight in the boat then a V8, plus the HP of the V6 was the same as the carbed 5.7
Merc does not release torque figures but im sure the torque was not as much as the V8, but it pushes a 20 Tiara great.
BTW- the second time the boat went out it had a defective brand new trim pump, Mercruiser paid the tow and a new pump was put in hours and the boat was back in action.
Im told sometimes shavings get in the pump during the manufacturing process.
you can go from something like this:
to something like this, you should consider re-gelcoating the bilge while you have the engine out.