Well, the area you decide to till is entirely up to you, and will depend on the varieties and number of plants. Keep in mind that even if you are going to want an acre of pumpkin, the only reason to work a continuous area of soil is for efficient use of machinery. The only places that 'need' be tilled/turned (making conceptual room for no till gardening) are where the seeds will be sown/ transplants are to be placed.

Some people do plant in mounds and whether you feel the need might be best determined by your climate and terrain. Mounds are not strictly necessary, but can help with drainage. If you have a flat area that can flood or not drain well, and you typically get ample-to-excess rainfall, then mounding can help. If your area is slopped, or raised, mounding can be unwieldy and can be affected by erosion. Your plans may not include them, but people who garden in raised beds are essentially growing all their plants in 'mounds'. Spacing is determined by variety.

So, were it me, I would start by choosing my varieties. The ones you decide on will determine the amount of space you need, as well as distance apart. Bush types obviously need much less space, but some vines can be incredible growers.

I probably live in a more Southern climate than you at 5a (Tn.), and grow a winter squash called North Georgia Candy Roaster. I raise mine under row covers to keep the vine borer off them, but they, like many pumpkins and winter squash, are vigorous vines that prefer space. There are a number of Candy Roaster and Banana types that are fun to watch grow, as well as extremely versatile in the kitchen. They can be made into soups, roasted, sauteed, made into desserts, etc... The flavor of the one I grow is mild, but quite delightful. Some of these get to enormous sizes! Choose varieties that do well in your climate, and can reach maturity based on average first and last frosts for the number of days between planting and harvesting.

If you have vine borers, consider Moscata types. The staple, and probably the most popular, is the Butternut, but these too come in many shapes and sizes. They also have many uses in the kitchen, and fill your requirements for texture. Variety is always nice, and with seed catalogs now on the internet, it's more convenient than ever to find a huge selection from which to choose.

I would start amending the space now. Once you decide on varieties, then plan out where you want to plant. Start dumping organic matter in a 3' - 4' round heaps. If you have access to 'clean' manure (no herbicides), dump that there too. Basically, make compost piles where you are going to plant seeds. Because you have so much time before planting, the manure can be raw (uncomposted.) In the fall, turn the organic matter into the soil (add the types of fertilizer that need time to decompose if you wish,) then mulch with leaves. Let that sit all Winter. Then, in Spring, till in your mulch and fertilize... mulch again. Plant when your soil warms for the varieties you've chosen.

If you want to kill the grass in the entire area, you can either mulch thickly if you have access to such resources, or put down newspaper and cardboard. Wood chips and thick layers of straw/hay/grass clippings, leaves, etc... are examples of mulches that would be appropriate given the length of time between now and planting.

Oh, and a soil test from a reputable lab (like those provided by your County Agricultural Ext. - in the U.S.) is the best way to find your soil pH and get recommendations for what amendments are appropriate for your soil and conditions.

Hope that gets you started with ideas.

This year I planted several Japanese squashes and watermelon in my vegetable garden, which is about 30 x 30, along with tomatoes, cukes, other greens. Needless to say the vines are overwhelming. I am thinking that next year I will put all of the squashes outside the garden, on compost piles. Just let them sprawl away. Only thing is figuring out how to protect the fruit..