If I went from small block to big block in my 29 Sunsation would I absolutely need to cut new holes in the transom? Or could I have pipes fabbed up out of stainless or Aluminum with bends to compensate? I just don't like the idea of cutting new holes in the transom. I am happy with the 377 I currently have but a big block would be nice when that motor expires. Give me your thoughts.

The Mercruiser templates call out for Alpha or Bravo outdrive for the cutouts, not by engine. I am guessing if you leave the same outdrive you should be good to go, I am not by any means an expert on the subject.

You can definitely refit with different exhaust. Class 6 here on the forum could help you out with that.

These are the standard Mercury exhaust hole locations.

https://www.hardin-marine.com/Instru..._Locations.pdf

I will have to PM class 6 on that. I am running a bravo already with 1.65 gears I'm guessing those gears might not work for the big block though??

Plenty of Fountains and Formulas with BBC's run those gears.

What I would like is over 425HP next engine. I have thought of keeping it small block with a built engine and carbing it. Or going BB and it would be carb as well with the additional work to make the BB conversion. This may not happen for years but I think about how to go about more horsepower without braking the bank all the time so I'm ready when that time comes.

Nice...so I have a good chance of not needing a outdrive if I did this!

What do you mean good chance ? Tons of BBC single engine Fastech and 27-29ft fountains with single 454/502's came with 1.65's.

For someone like myself what is the difference between those two gear ratio's? Why would they use them gears? All other boats I have owned had the 1.50 gears (all 454's)...thought the 1.65 gears might have been based on the the small block for some reason. Its good to know they will work then!!