Originally Posted by slowmotion
If I remember corectly. Sometime in the mid to late '70s olds produce a "Windowed main" block. It was made with less material and you can't put alot of horses to it. The name Mondello has been synonmous with olds performance for years. You could ask them.
http://www.mondellotwister.com/
The blocks that Dennis is speaking of have the cubic inch stamped right on the side of the block. Nearly all Olds 403's and all 307's have windowed mains and have there CI stamped on the sides of the block. That is the give away on the window blocks. The window blocks are not preferred over the solid mains for all out racing, but both are fine for general performance use in daily drivers and drive to the track race cars.
Bruce