Yes, it's the exhaust that is loud.
I don't have a good enough bottom end to go 6000 rpm.. Cast crank & pm rods. I'll have to run it till it stops accelerating at some point. But I don't think the Mercruiser ignition will go past 5200, supposedly the spark quits at 5200 til the engine falls off to 4700.
My boat is pretty fat, so you can pass me by with yours! But now it's at least respectable.
I gotta sell my 350 Mag now.

TBIV has no rev limiter, That's what mine has and yours being an 87 should have too.

The engine that I bought it with is a 97 350 mag carb with TB V. I read somewhere that they have a limiter. Is that true?

Read http://www.boatfix.com/merc/Bullet/94/94_02.pdf

It looks like the GGB baffle is the one for my situation.
My pipes are 4" OD and get pretty close to the transom. The inner diameters are 3.5" to my memory, but I'm going to have to measure them when I go back to the boat. I'm thinking about whether to use 3.5" OD baffles to fit inside the ends of the exhaust risers. . My boat is up north and I'm home working.
I think the normal way these inserts are done is in the metal tips but my risers go back so far that I may need to put the baffles in the aft ends of the risers.

I have the newer GGB inserts, they help quite a bit but still have a good sound. Didn't affect the performance at all.

Also, don't discount the engine noise. After I put the ggb in, was still pretty loud. Working the sunpad some to adjust it to seat better helped. Now we can talk in a pretty normal voice until about 4500 rpm. After that wind and exhaust noise require raising your voice.

Also, I have a bimini and when it is up it reflects the exhaust noise and it is much louder.

I'm going to put a foam rubber strip along the top of the rear seat back where the sunpad hits as a first step, Then might do insulation inside the motor box on the rear of the seat back.
I'm tired of buying parts for my engine right now, but I suppose I can pony up for the inserts if it's still too loud. They are under $200. I just don't have my boat at home where my tools are and when I went to the lake yesterday I didn't even think to measure for the inserts.
My riser tails are so long that I have very little distance between the ends of the risers and the transom fittings. My transom outlets have flappers in them that I want to keep, so the inserts will have to be in front of there. I'm thinking to put them inside my riser tails.
I see they are manufactured with attachment points. I'd think you have to bolt them down so they don't rattle. apollard did you bolt yours in to your risers?

If you put them in the riser tails, do not put them closer than 18" from riser to manifold connection. This is for hp reasons, lol.

You do have them pointed out the back of the boat right. Just kidding.
My 598 at 800 + HP IS LOUD but I don't have to raise my voice that much to make my self heard.

Yes, I bolted mine in the tails. Bolt is actually a couple of inches outside the transom, inserts run from there forward. Last part of the insert is actually in the hose connecting risers to tails.

I will say this about the inserts - they need water to work best. Before the water hits the tails, it is much louder. Then drops to a nice mellow tone. If I ever loose water flow, I know it way before the temp goes high by the exhaust tone.