If my brother had bought a 261 back in 87 or 88, I would probably have it ...

(Of course I would be DOUBLE pizzed at him NOW!!)
The coupler is the rubber coupling that bolts to the crankshaft on your 460. The drive shaft yoke from the outdrive slides into the coupler....you want to be VERY careful with them because you now probably have a LOT more torque and HP now than you did before.
If you "spin" a coupler it won't be pretty.
(I sold my flywheel housing to a guy in Calif that had his coupler literally explode and destroy his flywheel housing and inner transom plate......the plate is available used but he couldn't find a flywheel housing anywhere.... and was just about to tell the customer to "junk the boat" because it wasn't worth repowering with a New Merc/Volvo $24K+)
Also, you need to buy or make (if you know someone with a lathe) an alignment tool to check/adjust the alignment before you install your drives. (if the drives are on and you didn't, you need to pull them and check it. mis-alignment will destroy a coupler pretty quick and you have to pull the engine to change the coupler.
1. They're not made anymore so they're not available (the OEM couplers).
2. There is a company in up in Canada,
http://www.performanceengines.ca/OMC-couplers.htm that will modify your flywheel and install a different coupler. They weren't around when I pulled my 460.
I also thought I had exhaust manifold problems (it turned out that I didn't) and tried to buy new manifolds.
Couldn't find anyone that sells them. (This was about 4 years ago).....The marine parts companies I spoke to said that no one makes them anymore. (I think someone is making manifolds now though)
You could use custom stainless steel jacketed headers (I checked on those too) Big Buck$.
Anyway,
I still want a 261 though! I want to put twin 383/Bravo III's in one!
Let's see pictures!
Cheers,
Rick