Based on one of my old shop manuals, Motor Auto Repair dated 1967, the 413 in the Imperial was offered with two different horsepower ratings. They list one at 340 and the other at 360 hp. Since they both list having the same single 4 bbl carb., compression ratio the only differences maybe the cam timing and or the size of the carb. Any good-looking engine in the correct scale and representing a 383 thru a 426 wedge should work out.