MDS Marinized Duramax's are sold through Performance Diesel in Texas. Very nice package, available from 350hp to 500. No need to wait, I believe they have then in stock.

Or just go with new Cummins QSB 6.7ltr at 550hp 3300 RPMs 1250 FT BLS torque you wont have trouble with service they are world wide and the motor runs like a beast ! With Arneson drives you should have a bullet proof package.

The Cummins were my first choice and we built the engine compartment based on the 5.9/NXT set up but that began to look like trouble. Now I'm not sure if I have enough length to accommodate inline six's with a transmission. I think they will fit but don't like pushing it that close, maybe extension boxes would allow me to move the mounts rearward a couple inches. I think the 5.9 or the 6.7 with the available transmissions are longer than the 5.9/nxt was. I rode on innovations test boat with their military version 5.9 and it pushed their old 42 fountain to 55 mph with the single engine, they really have their fuel systems dialed in, never saw or smelled diesel smoke

I run the new 6.7 in my 48' APISA based on the Larry Smith 43 with trimax drives and we are running 82 + MPH what are the dimensions of your engine room I can get you overall dimensions if you need.

I have the QSB 5.9-480's with Twin Disc 5061 gears and ASD-8's in my 40' Super Hawaii. 2009 vintage engines and gears. If you need installed dimensions on that package let me know. Boat runs 70 MPH at 12,000 lbs. Non-stepped bottom.

I'll be back around the shop tomorrow, I think I have 66" between transom and bulkhead
Duramax fits easy, looks tight for an inline six and trans.
My drives are plug in 6's with beefed up velvet drive trans, not having a driveline should help with the space