Not sure I would try a 73 Sport as the fronts are different between the 73 and 74-76's.
Another issue I found once the S&H car was in my hands is the upper body line on the door is too soft and needs careful sanding. The tail light bezel is a poor fit and need to be more flush all around.
If you use the S&H engine block and Monogram Nascar 351 cylinder heads, intake and valve covers it makes a convincing Cleveland or 351M/400. If going stock, a modified intake from an AMT Ford FE. The Fire Fighter Mustang has stock style 351 Cleveland valve covers