It's been a summer of spinning wrenches on the boat. After getting a 540 mpi built out at a bunch of rusty junk, adding a remote filter with a bigger cooler, thermostat and 10AN lines and remote fresh water cooling. I took the boat out last week to have the steering cable strip now this week I think I spun the drive coupler. After 3 hours of putzing around the lake at various speeds I was easing on the throttle as I left a no wake zone the rpms went up but the speed didn't. Its got a stainless prop so no prop hub , the only other thing could be the drive coupler, am I correct? I assume that i need the short big block one in steel preferably OEM. I thought the alignment was good but it was a little crusty before I cleaned it up before installation.

Some stainless props use a hub.
Does it drive the boat at idle speed but slips with any increase in throttle?
Did you smell burnt rubber smell?

If you can get back behind the top of the engine, you should be able to see
the input shaft to the drive, when looking down.

With the boat on the trailer, back it in enough
to get the drive wet, start it, put it in gear, look down between the rear of the
engine and transom plate and see if the input shaft is spinning at engine speed,
if it is, it’s not the coupler

If the splines shear (which happened to me) as opposed to the rubber failing , there's no telltale smells but the end result is the same.

Typically if rubber fails boat will move at idle.
If splines fail boat won’t move at all.

The prop doesn't have a hub, I couldn't sleep so I got out of bed and pulled it off. The splines look good. At idle to 1500 rpm it makes a few mph- enough to get me back to the dock. The back seat is out and the engine cover is off so it didn't build up a smell but I kinda rember that but rubber smell for a second. I'm thinking it's got to be the coupler unless it broke the end of the crank off and I'm the luckiest guy in the world. Now to find the correct part .

Sounds like rubber let go in coupler.

Part number 8M0098795. Short steel , sound about right?

Looks correct by picture. Double check with original engine serial number. (assuming you had a 454 or 502)

If you have a drive shower you may be sucking air causing cavitation..