I agree. The frame should be plenty strong. It's going to see a lot more power and higher speeds, but that shouldn't hurt it.
My first post suggested replacing it along with everything else, but if you can install a drivetrain and are happy with the current suspension, that only means engine/trans mounts and maybe some extra brackets.

I don't know how much lower you can go, since the tires fill the wheelwell now. Take a look at how you want the finished product and what it is going to take to get there.