Unlike the 351C, almost all 400 blocks used the large bellhousing bolt pattern used by the 385 family big-block and were typically equipped with the higher torque-capacity C6 transmission. There were a small number of 400 block castings produced in 1973 with the dual bellhousing patterns. It had the large bellhousing and the small bellhousing bolt pattern used by the Windsor V8 family and the 351C, though it was not necessarily drilled for both. These particular blocks have been dubbed the "400 FMX" by enthusiasts, though were never officially referenced as such by Ford. Most 400's also had unique engine mount bolt pattern but these 400 FMX blocks had provisions for both 351C-style and 400/351M engine mounts. For 1972, the compression was reduced through the use of dished pistons. The compression reduced again for 1973 and a new timing set retarded the camshaft timing 6° to aid with reducing emissions. Changes to the cylinder heads for 1975 to add the Thermactor emission system caused the exhaust port to be more restrictive than the earlier 1971-74 heads. The 400 was re-tuned by Ford in 1975 to use unleaded gasoline with the addition of catalytic converters to the exhaust system.