Correct, you will never get a fiesta using a 45 inch anglia back axle and 10X13's under standard arches to handle well with 350bhp in a rwd fiesta. (Well for a start off the axle will fail at 150bhp regardless before we even consider the geometry problems)

You need to have experience of building such as small rwd car with that kind of power to understand the handling problems. Dangerous is not a strong enough word to describe what you will have to drive!