My gen 6 454 mag mpi is currently getting rebuilt after after 3000 hours. I am keeping it stock. I have purchased all new coolers..fuel pumps...had smitty do my injectors. The shop did call me and recommend new heads due to some rusting around the valves which i believe occurred prior to me buying the boat 10 years ago. When i removed the manifolds and risers the previous owner went too long before changing them. I wanted to see if anyone had any advice on the stainless marine exhaust. My engine is in a 27 conch center console straight inboard and is setup with the 6 inch spacers between the manifold amd risers. I hate the design of the cast manifolds with 2 gaskets but I do change everything out after 4 years. I see stainless does make a 6 inch setup. Would you guys recommend spending the money or just change out the cast manifolds and risers every 4 years?

Are you in fresh or salt water ?

Saltwater

Bit of an aside: you got 3000 hours on a 454mag before rebuild?

I was thinking the same thing.

3000hrs is a pretty good milestone.

What is the cost in parts and labor to replace the cast manifolds every 4 years?

8 years = 2x replacement cost + added risk of leaking and bleeding sea water into the exh ports.

Stainless Marine are a great option, and if they happen to fail, they typically leak to the outside into the bilge, vs into the port and engine. Plus you don't have the labor cost every 4 years.

You may be able to get some used options as well.

Used to run our Merc 454s up to 4000 hours in the parasail boats before with basically no problems . Changed risers out every year and manifolds the second year risers again the 3 rd year than the 4th year the hours would be in the 4000 so we replaced with new motor package back when you could buy them complete for like 8900.

now the crappy gimbles were another story Used to do hourly beach pickups and the drives trimmed up thrashed the gimbals , they were so loose sometimes you would think the drive was falling off.

There are no bad stories out of Stainless Marine. It's a great product and great investment. Many folks who destroyed heads or motors, or were lucky enough to find the leak in their stock Mercury headers before such...
...have switch to Stainless Marine with very slight if any performance loss compared to the OEM header. If you are going from stock 454 manifolds, and considering Stainless Marine's setup, you will see a bit of a gain.
It's a fitting upgrade for the other work you are doing to the motor.

Yes 3000 hours on a 2000 gen 6 block. I bought the boat 10 Years ago from a close friend. I did manifolds and risers every 4 years. It's about 1600 in parts with oem parts with the 6 inch spacer. I decided to do a compression test this fall and I had a few cylinders that were around 90 with the highest 125. It still ran great but I wanted to do it over the winter so I wouldn't be out a fishing season up here in the northeast. I take really good care of it which is why im sure it lasted. Boat is propped right and I cruiser around 3300-3400 at approximately 23 kts. I think I'm just gonna keep changing out the cast manifolds and risers. Boat is a straight inboard mounted in the center of hull so I have approximately 2.5 feet of exhaust hose on a drop from the risers before it even enters the mufflers