"The exhaust flapper on any 28
"The exhaust flapper on any 280 lower was a factory option only.... this part by itself was never an "over-the-counter" part.
I'd NOT recommend the exhaust flapper in the exhaust bellows for any V-8.... too restrictive, IMO.
Now, you can modify a 290 exhaust flapper to work on a 280 lower unit. Part of this involves removing the raised portion (the bump) of the aluminum housing. No big deal!
This is because the 290 flapper does not accommodate the radius (the bump) in the aluminum housing.
The 290 flapper is straight across, so to speak!
Some slight cutting of the 290 flapper needs to be made where the two halves meet (the 280 was one piece and covered the bump).
The plate and rivots may not align properly with the 280 original rivot layout (I don't know as I have not retro-fit one that previously had a flapper on it)......, but I'd rather see the flapper on the drive than the flapper in the bellows.
(maybe punch new holes in the rubber and use the 280 plates)
Again, I'd do this for any of the larger engines!
4 banger????.... flapper in the bellows is OK!
If you do this mod, you've got a choice between aluminum or SS rivots. Either way, you've got dissimilar metals.
Of the two, I think you're best off to use the SS, IMO.
"