Street Rodder magazine had a article on this some years back.Because the mustang II carried 65% of the weight on the fronted the 4 cyl spring should work well.BE careful of right heights.Mt friend has a 40 ford 4 door sedan SBC,turbo 350 we put a mustang II frt end on it and it worked very well.He bottom out the trans pan on a speed bump.After changing the pan he raised the ride height by cranking up the coil overs. After this it would start rolling from side to side on the freeway.After some research we put a sway bar on the front and it fixed the problem.