You may find one of those after market Mercuiser repair manuals helpful. The
ones I had contained charts listing all the gear ratios for each engine
type. Find your old engine and your new (proposed) engine and see if they
have the same or similar gear ratios. This give some preview of any mismatch
between old outdrive and new engine.
The swap will be expensive unless you can find some used parts. You will
need a new bell housing, and probably the drive coupling on the flywheel. If
FWC then you will need and exchanger that will match up with new engine.
Then you will need a complete engine ,all accessories, flame arrestor and on
and on. Plan on sending a bunch unless you find a complete used unit then
you may be buying some thing that is not much better. I would seriously
consider a rebuild unless you are really dissatisfied with the original
setup. With the rebuild you don't need anything much except mainly internal
engine parts. Still this is an 83 and it is still running, looks like you
and the boat are doing well together as it is?

--
Ron White
My boatbuilding website is:
www.concentric.net/~knotreel