I think Mustang springs are much shorter then a half inch. Try measuring your old springs and the new ones along the arch not directly eye to eye. You may need to put a small load on them. It may help to bolt the housing to them. The U-bolts and shock mount plate may give a little squish to them.
You are likely to need to load the spring some to flatten it slightly to get the 1/2 inch to line up shackle. When I changed rear springs in my 68 I did it with rear end in place by doing one side at a time. I used a jack on the side I was replacing to put some load on spring. Without rear end likely need to block rear of spring and carefully jack spring in middle to line up shackle. It shouldn’t take much to get a 1/2 inch- just be careful with block placement and amount you jack. A small bottle jack should be all you need.
By placing something like a block of wood ahead of the shackle when you jack up the spring it will keep the spring down off the frame rail and allow you to connect shackle to spring. The process of loading the spring “flattens” the spring and will make the spring slightly longer by taking some of the curve from the spring. Without a block between spring and frame, the spring would just push up against frame and you couldn’t attach shackle. Always use care when jacking/blocking so the jack/block/ parts don’t come flying out. To get a 1/2 inch on a long leaf spring it should not take very much pressure from the jack.