Your best choice is to pop for a commercially available Mustang II crossmember for your chassis. It may cost you a few bucks but will save you countless hours of fabrication and hard work.
You already have the most expensive components, "A" arms, spindles, brakes, strut rods etc. so the only real expense will be the crossmember. This will keep you from having to slice the frame up since it's specifically designed to work with your frame and not replace it. There are also upper "A" arm kits that will keep your suspension from hitting the fenders if your willing to spend the money, otherwise you're going to have weird bumps in them.
Think safety and don't go hacking away on a perfectly good frame unless you have to. A quality crossmember installed properly will let the car sit right and in the end will cost you less in time and probably money.