The big expense is the upper trunnions themselves. I think they run around $150 each. The new ones have urethane bushings and grease fittings, they will last another 50 years easily! the alternative is to cut everything out and pretty much build from scratch. Mustang II components don't fit well, no one makes a kit for these. I've seen one or two done well, but it's a lot of work and cost. $500 in parts will rebuild the entire front end and you'll be set for years to come. A Mustang II kit is $1500 at a minimum for a basic, stock parts kit. Then you have to make it fit!
The hardest part is removing the springs. This is also the most dangerous part. The spring seats have "ears" on them. Get a piece of 1/2" wide, 1/8" thick strap steel. Bend a hook in one end. With weight on the front wheels measure the distance between the holes in the ears at their closest points (top to bottom). Cut the strap and bend a hook in the other end so that the hooks are that distance apart. Jack the car up and put a jack stand near the lower ball joint/tire, then take the tire off. You should be able to pull down on the fender and connect the hooks (you need to make two!) in the ears. Disconnect the shock and SLOWLY jack the car up, watching the spring. As long as it stays straight you are fine. If it looks like it wants to bulge out or get crooked in any way, STOP JACKING and try to correct the problem. If it won't come out straight you will probably have to remove the fender and use a scissor type strut spring compressor.
Assuming the spring stays straight, continue jacking the car until the spring is loose. At that point VERY CAREFULLY pick the spring up and lift it out. You're working with a loaded gun there, so act accordingly! It is VERY IMPORTANT THAT YOU NOT HANDLE THAT SPRING IF IT IS CROOKED. If it pops out something is going to break, and it could be your head. I mean as in skull popped open, jaw broke, broke arm, etc. YES, IT COULD KILL YOU!! I set the "loaded" spring carefully on the shop floor under the front floor of the car, which will keep it under there if it comes loose and you won't kick it over. There are holders made for this job, but the simple hooks work as long as the trunnions aren't so bad that the spring won't sit straight in the hooks. There is no safer way to work with those springs, except possibly removing the fenders and using the scissor type strut compressor. Just be careful! I've done this many times, but act like I'm an explosives expert moving a bomb every time! Oh, the engine has to be in the car for this to work too. The hooks don't compress the springs any further than normal loaded weight, unlike the factory hooks, which compress the hell out of the spring and REALLY load it up and make it dangerous! They are made in such a way to keep it from popping out, but it's much safer using the hooks and not compressing the spring further.
If you need to remove the hooks to change the spring use the car to compress the spring. Put the spring under the "frame" rail under the floorboard and set the car down on it. you might have to have someone press down on the front of the car to release it all the way. Then just jack the car up and pull the spring out, reverse to "hook" the new spring. I suggest you just do one side at a time to reduce handling the spring. The more you handle it, the more likely something will happen!