I have an 85 Lesabre Collectors Edition coupe, which came with an Olds 307. I did an engine and tranny swap with an 76 Olds 455 and Th400 several years back. The engine was never rebuilt and while it served me well, it's time. I've been putting together parts and I'm getting ready to rebuild the engine this spring. I have a nice set of "C" heads, edelbrock intake, Holley 750 and various shiny bits. I'm not sure which direction I should take as far as exhaust manifolds? Currently, I have the stock cast iron manifolds that came on the motor, which was from a 76 olds 98. I've used headers in the past and disliked them due to them rattling loose, burning plug wires, rusting, ect. I've been thinking about trying the reproduction 442 cast iron manifolds and having them ceramic coated. I'm hoping I would get the performance advantage I'm looking for, without the drawbacks of headers.

I'm wondering if anyone would have any helpful advice in the matter. Will the cast iron manifolds be the solution I'm looking for? Will the exhaust outlets allow me to bolt the manifolds to my current duel exhaust set-up without needing modifications from my exhaust shop? My target HP range is around 400, will these manifolds support this application?

 

Stick with the 455 manifolds as long as they have the center divider and the original 455 application was for dual exhaust or get a set of 400/455 W/Zs. Remove the stove flap in the drivers side and weld up the holes. Install heat riser block offs in the intake too. Then install a mandrel bent 2.5" system with some high flow mufflers and forget about it...unlike headers.
Im not sure your head pipes will line up. You will likely need new head pipes. They are not too expensive. Fusick and Year One and others such as Gardner offer the complete system as well as individual components.
The 307 manifolds will not work. You can have the W/Z manifolds extrude honed then ceramic or high temp powder coated.

 

Thanks for the input droidsmorland! I currently have 2 1/2 duel exhaust installed with a set of turbo mufflers hooked to the 76 455 manifolds that came with the motor. Originally, was able to use the stock 307 exhaust when I first dropped the 455 and everything lined up. Obviously, it wasn't the best set-up, but it did buy me a couple years until I was able to afford the duel exhaust upgrade. My installer capped off the center crossover dump and used the rear dump for my installation. I knew it would be a long-shot to have everything line up again, but it would have been a nice bonus. The heat riser valve was another thing I wasn't sure about, so thanks for that as well.

 

This is the manifold set I've been contemplating: https://www.opgi.com/cutlass/C240268...FUa1wAodQ3UFXA

 

Yes the 69 and up WZs have the center divider. That is the more desirable design. Its worth a few ponies. The earlier design XYs are very similar less the divider. Thats the only real big difference between the two designs. You may be able to have a good pipe bender graft the proper head pipe into the existing system.
Now the problem with the C heads is their center divider is recessed, (68 for sure not 100% sure about the 69 but I believe so). So look at your Cs. You need to have the head divider welded and milled to bring it level or have the manifolds center divider welded proud to seal properly. Ive see it modified both ways. I prefer to do the head. The metal is more conducive to the heat cycling and is a more durable mod. If you dont want to go through all of that use the XYs. You wont lose a ton of HP using the those. It wont hurt to use the WZs with the recessed head its just not taking advantage of a true separated port.
Make sure the intake is "matched" to the heads. If the heads have ever been milled it will affect the intake valley angle and the final compression ratio. You could run into intake leaks due to improper head to intake interface. Also note no gaskets are used with either exhaust manifold design. The head pipes are swaged and do not use a dough nut, its metal to metal. Apply hi-temp copper anti-seize to all exhaust hardware. You'll thank me when you need to unbolt things down the road. If you use anti-seize back off slightly on the torque spec to compensate if you use a dry torque spec (minus ~10-15%). Re-torque after several heat cycles.

 

I would suggest the X and Y versions as originally used on all 1973-74 U and V code Oldsmobile 455 A body cars. Your 85 B body is the same chassis as the 73-77 4 door A body cars.

And there is a set for sale in the parts for sale section.

EDIT. As Joe ponted out this should be X and Z

 

The 73-74 cars used the X/Z pair, actually. That's exactly the same 402294 "Z" casting as used on the 69-72 cars. The 412287 "X" manifold used on the 73-74 cars is dimensionally and functionally equivalent to the "W" manifold used on the 69-72 cars, including the individual runners. Either the W/Z pair or the X/Z pair will fit and work just fine, assuming you want true dual exhaust manifolds on this car.

 

The passenger manifold will exit at an angle instead of straight down so you will need a new head pipe for that side. I think the angle on the drivers side might be the same as the manifolds that you are currently running, but I'm not 100% on that one.

 

Brain fart. I meant X and Z. Not sure why I typed Y. Forgot to proof read.

Regardless the 73-74 BBO version is the ticket.