Look at the pictures of the different transmissions on the Novak site.

http://www.novak-adapt.com/knowledge/sm420.htm

Just behind the clutch splines is the bearing retainer. the long skinny part is what the throwout bearing rideds on. Behind the long skinny part is the indexing circle. On the sm420 and many other gm transmissions that circle is 4.686 inches.

The sm465 replaced the sm420 and was indexed to 5.125 inches. The bellhousings for a sm420 and a sm465 are virtually identical, but the sm465 bellhousing will have a larger diameter hole to accomodate the larger diameter of the sm465 transmission bearing retainer.