Replacing the riser gasket

Sounds about par for the course.... Guessing you can't get a boxed end wrench in there???? Mix up some Tranny oil and acetone 50/50 mix and soak it for a bit. Looks a little rusty in there and I'd imagine the bolts haven't been moved since they were originally installed.

If the bolt is still being stubborn then soak it some more with your mixture then run the boat for a while to heat up the manifolds (or use a heat gun) Metal swells when warm so that might loosen the grip the manifold has on the bolt. You could use a torch to heat up the manifold but then you'll ruin the paint on it but a rattle can of black high temp grill paint can touch it up.

I had the same problem when getting my manifolds off. If you search my name in the forum, you'll fun a thread with a ton of techniques and failures regarding manifold bolts.

I broke 3 of 4 of my bolts from the risers. Fortunately, it didn't matter because my manifolds were cracked anyway. The technique that yielded the BEST RESULTS :

1. Lots and lots of Automatic Transmission Fluid mixed with acetone.

2. Weld nuts on to the existing bolts. The heat from the weld will work miracles.

3. Use an impact wrench to back them out.

This combination saved me LOTS of money and time.

Trust me, you don't want to spend all your time dealing with machine shops.

Also remember, when you finally replace the rider gasket, one of the water jackets are intentionally blocked (should be the rear most if I remember correctly).