I will be putting a 500 in my 77 Deville this weekend. I am going to cut the bolt by the #1 cylinder enough to clear the 500's exhaust manifold so everything should slide right in. My only concern is the exhaust pipe. Will the pipe on the Deville in its stock position conect directly with the 500? I'm hoping to make this swap as easy as possible and don't want to have to take the manifolds off the 425 and put them on the 500.
I'm not sure if it will line up perfectly of not.
Make sure, before you use the 425 manifolds, that they will completely cover the exhaust ports on the 500 heads.
The 425 manifolds are more restrictive than the 500's. If it was me, I'd modify the exhaust pipe (or have one made at an exhaust shop) to work with the correct manifolds or have duals put on it.