So I know the 425 exhaust manifolds are restrictive, while the 472/500 aren't anything for performance, they do flow better, I was wondering if the exhaust manifolds would bolt right up and have no issues running them in the '79 DeVille chassis.

IIRC they'll work but you'll need to cut some excess thread off of some bolt that's in the way. I don't know if I'd even bother though, since the stock manifolds suck and since it's not too hard or expensive to do headers.

well, i heard they flow alot better then the 425 manifolds....

I would love to throw headers on there, but there are no after market headers that will fit in the '79 chassis (C-body) so clearance is an issue.

basically any exhaust work I do (manifolds/dual exhaust) I want to be able to use when I get the 500.... so I wouldn't mind spending the money on a pair of headers for the 425, if they'll bolt to the 500.

Hey Nightwolf, yes the manifolds will bolt right up and like davesdeville said the only clearance problem that you will have is on the passenger side in which you need to trim about a half of an inch from one bolt I believe the bolt is part of the suspension setup in the front. It wont affect anything by cutting off part of the bolt. That is what I am going to be doing when I swap in my 472 into my 78 fleetwood. Just be careful when you're swapping manifolds because those bolts manifold bolts snap pretty easy. The best time to pop them off is when the engine is warmed up.
Good luck - Justin

hmm, in that case I'll have to pick a pair up next time see them... cheap way to get some better flow whenever I get around to getting dual exhaust.

I think Al at MtS told me that the 425 manifolds flow the same as the 500's except on the #1 cylinder. I used the 425 manifolds on a 500 swap into an 82 Fleetwood with 2.5 in duals and it seems to flow ok. After I put the engine in I hooked up the stock exhaust for the drive to the muffler shop. I was a little disapointed with the performance. After getting the car back and mashing the pedal it was like a different car. LOTS BETTER. I would rather have headers but I don't know of any drop in replacements. Has anyone tried using some type of headers with caddy flanges and had any success?

Dunno if it will work with 77+ RWDs, but on my 75 I should be able to use Hooker 2445 (I think that's the part # anyway) BBC headers for 70-72 Monte Carlos, which run $150 for the non-ceramic coated version. Of course using Caddy flanges.