I have amended clay loam soil using sand many times, and it has always worked well. I have also encountered situations where the topsoil was removed, and only pure dense subsoil clay was left. I don't see any point in trying to amend pure clay. Just pile good topsoil on top of it. I have also heard it said that there are certain clay soils which react very badly to sand. I can't prove or disprove that.
You say you are planting melons for the ninth year. Do you mean in the location with the clay soil? If melons grow in that soil, it can't be pure clay. It is more likely a clay loam. Has anyone in your area used sand as an amendment?
If you can't get a good answer, this is what I recommend: buy a bag of sharp, coarse sand ($3) and spread it 1 inch deep over a small section of your garden. Till it in. Live with it for a while. Plant stuff in it. Let it get wet and get dry. See how it goes. If it works well for you, do the whole garden, maybe next year.
If your soil cracks when it gets dry, it probably has high clay content. If you till in enough sand, the soil will not crack, it will dry and warm up earlier in spring, root growth will be much better because of improved soil aeration, weeds will be easier to pull, etc. I find that plants grow so well in loam or sandy loam that I don't even bother gardening in clay loam anymore. I just go right ahead and amend with sand (and compost). Melons and peppers seem to really like sandy soil, in my experience. If you are planting pot-grown plants, their roots will grow into a sandy loam soil much more easily than a clay soil, because the structure and aeration of the sandy soil is not much different from the structure of the potting mix.
Sand works immediately and permanently. As soon as you are finished tilling in the sand, the soil is ready to grow crops in. Organic matter will improve the structure of a clay soil, but it can take a long time. Sand is permanent because it does not decompose. Once you till sand in, it is there forever. So I start my soil improvement program with sand. After that, I add organic matter, lime, superphosphate, or whatever else the soil needs.
Q