Check out the thread titled 3.8L-4.2L. This might give you an idea of installing a 4.2L engine in the car instead of a rebuild. Can't help with any cost estimates, just another idea on what to do with your money!
I would personally see how much a New 4.2L shortblock costs from your local ford dealer and bolt a get your stock 3.8L heads ported and a 3 angle valve job done and bolt them on the 4.2L shortblock and drop it in your car. 3.8->4.2L = extra 24 cubic inches and 30ft-lbs of much needed torque.