I would have to say a big no on the frame change especially a unibody . A forty ford body in its self is not very sturdy. The original frame is designed to fit the contour of the bodywhich gets its strength from the frame. Although it is not impossible to use a different frame such as a ranger as mentioned I guarantee you will spend hours upon hours to get it even close to working . I would suggest an original frame with perhaps a mustang 2front suspension and a leaf spring rearend kit , both of which can be bought for a reasonable price. I just did this and can tell you that even with this set up it is no easy task . You have very limited clearances at the firewall and at the radiator when you change engines ( which I suppose you will do) . Believe me you will be much better off with the original frame .