Howdy,
Well, the coupler problem for the 460 is actually quite unique to the 460 since OMC only offered them from 87-90 and then discontinued them permanently.
I had a quick look at Doug Russell and got the part number for that 89 5.0/5.8L coupler (3853862) and not only does Doug Russell have it but you can get it at Go2marine and a bunch of other places. It's about double what a coupler for a Mercruiser would be though...
You're also looking at the most problematic of the Cobras. (some imply that the early Dog-clutch Cobras were partially responsible for OMC's demise) Read here:
http://www.hastings.org/~stuart/cobra/If you MUST have an old dog-clutch Cobra with a Ford Marine engine, (Both of which are not manufactured anymore) You should get an OEM service manual and become familiar with that link above.
Make sure that you can get common wear items like the coupler, exhaust manifolds and riser/elbows. It also might be a good idea to find someone that can install/adjust a lower shift cable since maladjustment will cause severe vertical drive damage/destruction and some internal parts may not be available anymore for certain dog-clutch Cobras.
My suggestion? If you have a choice (I.E. someone is NOT holding a gun to your head) Don't buy a boat with an OMC Cobra inside.
In todays economy, there's literally thousands of boats out there with Volvos & Mercruisers for sale and they're getting cheaper.....(Volvo and Mercruiser are still in business too!)
Regards,
Rick