To give a picture of the situation of the level of cost to the project.

I am buying the boat without engines and drives:

I have to pay around usd 6400 each small block (used 2000-2002 mod) around usd 3700 each bravo drive(used same year model). Aprox usd 20000 for the complet twin instalation.

Big block 350-400 hp usd 8500 and usd 4000 for the drive
Aprox usd 12500.

Rebuilding costs from twin to singel aprox usd 2500

Cosidering then the fuel cost usd 2,3 each ltr. and service
costs. I wil save a lot of money in a singel instalation.

The question is the loss of speed and preformance of the boat.

Sell that nice boat and buy what you want. The cost difference will be less than the cost of the work.

You can buy a lot of gas with the money you are going to spend on that project.

I have a friend that has a 28 ft. 1995 Wellcraft Martinique with a single 454 carb that burns the same to more fuel than my 38 with twin 502 mags at cruising speeds.

Had a 33' 10' wide cruisers inc, (14,500#s on the trailer) twin big blocks 55MPH a real dream of a boat. Went to a 31X10 wide cruisers inc single, took it to 560HP with a procharger (did it twice as I blew the first one) lots of $$$$ 48MPH 50 the day it blew. 11,500#s on the trailer and it uses more gas than the bigger twin. Why run the piss out of a single when you can run twins in the non-destruct range for less gas with more space?

I am in the same situation. I have a 72 Cig with twin 165's and a drive spacing of 25 1/4. When I have to repower, I will need to do transom work regardless of whether I op for a single or twin small blocks. I my case I will probably go with a single 496HO.

Doesn't it come down to what you want out of the boat, and what you're willing to give up to get it?
My suggestion, from a performance-whore perspective, would be to build a twin using SBC's and Alphas.
- the Alphas you can buy new on eBay for $2500 each
- the SBC's and parts are available internationally
- the ongoing HP vs gas$ isn't going away, so have a mild setup for cruisin, and radical for when JoeBlow pisses you off and you want to show some muscle.
Of course, this is from my perspective..

How about this..........

Glass the transom in, put a bracket on there with 2 outboards???

I have 2 really nice mild & fresh 280-290 HP small bocks Completly ready to drop in , they have Everything. I just listed them in the classifieds take a look & let me know if you are interested.

I would sell the twin and buy a boat with a single. You could do it for less money, time and frustration and have a better boat in the end.