John
It does not list the H.P. ratings on the Merc sheet from Holley.
It does list the H.P. on the same style Holley information sheet for the Ford as 270 OR 285.That is Holley carb spec sheet ,Form No. F-136,Issue No.1 (1-57)
Remember these are carb spec sheets.Its Holley Form No. M-210,Issue #1 (1-57) 1957 Mercury,Maxium Duty Kit,8 Cylinder,Stand. & Auto. Transmissions (312 cubic inch engine).
Can send you a copy of either the Ford,the Merc,or both sheets if you like 
As I previously said,its all of the same carburator specs and part numbers as for the "E" code Fords.
Oldmics