TripDeuces

I know this subject has been overworked and beat to death but I'm still a bit confused so here goes.

I have a 66 Cutlass that originally has a 330 in it and I'll be replacing it with a 455. I know the bottom ends of the block are all the same for any size Olds because any oil pan will bolt up. I've been told that as long as I use the same motor mounts and frame mounts as a pair the motor will fit fine.
So here's my question:
Why did Olds make two diff motor mount and frame combinations if they fit any size block?
If I use the small block mounts on my big block will it sit higher or lower in the frame?
I know the oil pan rails are all the same but are the motor mount holes on the block in different locations?
Since this is a big block do I use the two rear motor mount bolt holes?

I think that covers it, thanks for any help here.

TripDeuces

After reviewing many of the related threads on this subject it appears I should just stick with the small block listed mounts and put them in the back two holes on the block. I understand the confusion now. It seems to be with the aftermarket rather then Oldsmobile. Thanks all.

svnt442

Correct. If you look at the two types of blocks they use the same timing chain and cover. This also means the water pump is in the same location on both and so will the fan. So using the SBO mounts on a BBO will have the block sitting the same height in the frame.
They had two different types of mounts because they incorporated a "locking" mechanism in the BBO mounts so the engine wouldn't hit the hood is the drivers side mount broke.