I recently scored a set of like new Manifolds with the 4" risers (I believe all '87 and newer had these). My boat currently has the 3" risers on them now. What would it take to change to the 4", is it as simple as using the 4" coupler? Are the angles different?

should be no problem some of the older 3 inch models had a round y pipe instead of the oval. as long as you can come up with the right plumbing it will work. or if it were going out through the hull you would need new exhaust tips.

It is still stock and goes into a y-pipe. I had thought about going through the hull. Do I need to do anything special, or is it just the block off plate and drill some holes?

just a block off plate. and the holes in the transom for the exhaust thru hull tips. you can also plug your y pipe with wood blocks and accomplish the same thing with out pulling the engine.

Cool, I think I might just do that. I've already got the motor out for rebuild so I can just pull the y-pipe and put in a block off plate. I guess I should buy some tips with flappers on them.

If the exaust comes out below waterline when sitting in the water, yes you need flappers...