Yes, a Pontiac 455 block stamped "Auto" can be used in a manual transmission build. The "Auto" stamping refers to the fact that the block was originally intended for use with an automatic transmission. This typically means it has provisions for an automatic transmission's bellhousing bolt pattern. However, this doesn't prevent you from using it with a manual transmission.
You'll just need a different bellhousing (one designed for a manual transmission) and potentially different flexplate/flywheel and associated components. The block itself is perfectly compatible.
