I have owned a 1965 Chris Craft Seaskiff with twin FWC 327 V-8 engines since 1986 - 20 years (just sold, thankfully, as owning 2 boats is not twice as much fun as owning 1), so I've had a little experience with risers. I agree that the long studs can be a pita when it's time to remove them, but not neccessarily from crud, although that does contribute to the problem. One of the obstacles to removing them is gravity - a fact often overlooked by mechanics. Those suckers are fairly heavy, and their weight pressing down on the studs impacts their removal. What I have done is to place a small bottle jack under the aft end of the risers, and apply slight upward force on them. Taking the pressure off the studs can make all the difference in the world.
Next take the slimmest screwdriver you have, and use it as a wedge between the riser and the gasket, and tap it down into the crevice. Try to avoid the manifold side of the gasket. If you chip anything in the process, you want to chip the riser that is being replaced, not the manifold. Once you get a gap opened up, try to pry off the risers, rather than pound them with hammers. If they absolutely won't budge, apply a bit of heat, but do so judiciously, as it is a gasoline boat after all.
Another approach is to lock two nuts on the end of the stud, and try to turn the entire stud within the riser, and actually unscrew the stud from the manifold. This doesn't usually work, but hey, it's worth trying.
With respect to the frequency of replacement, I only replaced mine as they failed. I would only recommend this if you are very much "in tune" with the operation of your engines. They WILL warn you that they are failing before they cause catastrophic damage, but you have to be listening.
The first warning is a slight skip - a missing cylinder - on either cyl 7 or cyl 8, depending upon which riser is failing. This occurs at startup, and clears up quickly. The engine starts running on all 8 again, and sounds fine. What has happened is that a small amount of water has worked its way past an open exhaust valve and into the cylinder. Once it is expelled by compression, the cylinder begins to fire again. To verify that this is the problem, let the engine sit overnight. Now, before starting the engine first remove the coil wire, and then remove the sparkplugs from the rear 2 cylinders. Crank the engine and observe to see if water is expelled from one of the cylinders.
On rare occasions, I've had water accumulate in cyl 6 or 5, if it happened that the exhaust valve on 7 or 8 was closed at the time the engine was shut down. That usually doesn't happen until the riser is leaking quite badly.
If left un-corrected, enough water will accumulate to prevent the cylinder from firing at all. If still left uncorrected, enough water will accumulate to bend the connecting rod on a compression stroke.
Good luck with the project.
Bob
Sorry.
