The L72 engine was rated at 450hp for a short time at the beginning of the 66 model year and then "downsized" a little. Most assume it was for insurance purposes, or maybe to balance out the presentation between the L78 and L72.....as for real horsepower....blueprinted L72 with tuning and headers will probably produce 450hp or more...the L78 version can make 415-425 with similar prep.
I seem to remember the article about the Harrell prepared 427 cars being rated at 450hp...you can have any horsepower you want for about $3.00...just buy the sticker!
wilma