Even with those added costs, the Roush Stage 3 Mustang should slip nicely between the Boss 302 and GT500’s respective $42,200 and $59,200 price tags, which is fitting for the upgrades it offers.

Ford has begun testing prototypes for the next Mustang Shelby GT500 on public roads around its headquarters in Dearborn, Michigan. The prototypes are based on the Mustang Dark Horse but wear ...