just starting looking into extension boxes,,,so is my thinking right here,,,if i go with a 12in imco box,,,that puts the x down about an inch,,,so i would want a -3in shorty,,,,and if i didnt put he boxes on then i would go with a -2in shorty,,,in either scenario,,,the x would be roughly the same,,and by x,, i mean center of prop shaft

Probably be hard to get on plane with -2 shorty without the extension box. It does lower your props a little, but your in alot cleaner water. To go -3 you'll have to buy a IMCO SC Upper.

I'm doing the boxes and -2 shorties this winter. Depends what you current prop height is.

my 24ol was deep at 7'' I put the 2'' shortie on and my boat road better flew level,and pick up 8mph with carb change and shortie. with your boat the imco standard boxes with 2'' shorties might be just right since yopr boat is a 35ol or you could put the 3'' raised box with your drives but you need to raise your exhaust holes and adjust the front of the motor I heard. I bought a konrad ace comes with a box and will put my prop shaft below height at 3 1/2 '' on my hull or the new one I am looking at for starters, then I could add a 1/2'' spacers if needed. I plan on running a 5 blade I have.

What is your current drive depth? Just curious mine is at 8" and I would like to raise it.

if everything works out,,with my extension boxes in neutral and -2 shorty,,what rpm increase might i expect????if the boxes put the drive 1 inch lower and the -2 shorty bring it up 2inch,,,i would be 1inch higher than normal,,running twin 525engines 30 labbed props 5100 rpm

If you are going to only net + 1" on prop shaft height, my guess is + 2 MPH....lot's of $ for a few MPH. Where will this put your prop shafts in relation to the bottom?

There's the magic question

Why not just go with -1 lowers? You have a notched transom already right? The cost of pulling engines, boxes, shafts, install and steering mods would cost a ton.

hope to get some benefit for running in rough water conditions from the extension boxes also

The attitude of your boat will greatly improve with the extension boxes and depending on your current setup meaning prop height you might be right where you need to be with -2 shortie and neutral Imco box. There is also the option of a Latham box they have a 7inch extension box although it's REALLY pricey. I personally would stay away from the Stellings although many people are running them without issues. I just don't like not having the cupler and the drive taking all the shock and vibration. Good luck on this as it's exactly what I'm doing this winter to mine and I'm hoping for if nothing else better ride in rough water conditions. As I stated in post #2 if you want -3 you'll have to go with a Stellings box or buy IMCO uppers to achieve this.