Had some motor problems in the family boat this past weekend and now I'll be replacing a short block... It's a 1987 model year boat, but I'm assuming that it has a 1986 motor in it. According to the casting number on Mortec, it could be 1986-1988, so I'm figuring on 86. So that means hydraulic flat tappet...

It has the stock Merc manifolds/exhaust running through the prop.

I was thinking about using Comp cams 12-232-3 (grind CS XM 256H-12). Or possibly even 12-236-3 (grind CS XM 262H-12). Would reversion be a problem with either of these? I'm not looking for a huge power increase here. That's what the (still unfinished) Pantera will be for

XM256H = 35* @ .006 and -9* overlap @ .050
XM262H = 41* @ .006 and -3* overlap @ .050

I don't think there will be a reversion problem with either cam especially the 256H. If you do opt for the larger 262H then pull a riser after running and visually check for reversion in the manifold just to be safe.

What compression? WOT RPM range? Intake? Heads?

If your staying around 9-1 with stock heads and intake and are not going to go above 5K rpms shoot for the 256H. That should make a better running useable powerband for the boat.

it's for the family cruiser...

compression will be less than 9:1 (~8.7)

the motor is stock (heads, intake). I just figure that it will be easier to change the cam while it is apart in my garage

If your looking for a comp cam kit (cca-k12-236-3) new stll in box i have one for $280 shipped, bought it for a boat but never installed it.
Jim