Aren't tillers wonderful????
I've used sand in the past but usually on smaller areas than your 50' x 75'. It'll take quite a bit of sand to make a difference if your clay is real bad. Just add anything organic and till it in ... the more the better. If you can compost it, better still. It just depends what you have access to. I used to find that riding stables were a great source of material. They are usually only too happy for you to get rid of some of their stuff from cleaning the stalls, manure, straw and all. It's great stuff once it's composted. Around where I am, dairy farms are another good source although it needs to be composted ... too many weed seeds otherwise. I just use anything I can get my hands on. If it's fresh manure, I usually compost it but if I do add it directly to the garden, I usually do it in the late fall after clean-up, and let it break down some over the winter before tilling and planting in the spring.
Gary, I never have found a good solution to the trench or pile problem at the end of rows. I even tried tilling in a circle once. Now, after I finish tilling, I make one last pass, tilling in one direction. At least that way I end up with the trench at one end of the garden and a pile at the other.