At a guess, I think a Plexi style pedal might help to get you closer. There are quite a few. The Marshall Guv'nor is one of the earliest ones, and they're reasonably affordable. There's just a certain combination of compression, harmonics and high end spank that gives Marshalls a distinctive sound. I'm not sure what the capabilities of the Mustang are that way, but you're probably not getting anything close to that from a DS-1 or Ultra Metal type distortion. "Iron Maiden" and "JCM 800 2203" have a lot in common.