Bad news- have to pull engine again. Oil is being sprayed on side of bilge area. Suspect a trim fitting or hose let go. Trim reservoir was nearly empty when checked. The last time this sbc 350 was rebuilt I noticed the coupler was rusted and pitted to the extent of holes were appearing. The coupler is mounted on a triangular plate and held on w/ 3 bolts attached to the fly wheel. Can I use a different coupler which is small and compact retained w/ about 8 bolts. First of all the small one is
cheaper. But can it be done? Running a 1988 alpha. JIM ps. Modified motor 400 hp.+

Don't know much about Small Block/Alpha setups but would suspect you would have to go with the stock Mercury part. Call Bam Marine at 954-545-8681.
If there are choices, go with the stronger and make sure your alignment is correct. That will waste the coupler quicker than you would think, if that is not correct.
Welcome to the board!

I don't think that the strength of the coupler differs with the size. Couplers only come loose from the rubber after they rot (age). Too much horsepower will usually destroy the splines not the rubber. I believe that the extra rubber (larger diameter couplers) were designed/installed to reduce driveline noise from being transmitted into the boat.
Sincerely
Dennis Moore