If you really want to grow a giant pumpkin, you need to be prepared to grow just one per plant. i.e you will grow the first flower that forms a pumpkin, and take off any succeeding flower.
But the plant needs to grow lots of leaves to make that huge pumpkin for you, so no, you should not prune it. It will form a long trailing vine, so plant it where it has room to sprawl. Some people grow the vines on a fence so they're not in the way, but the pumpkin will need to sit on the ground if it's going to get big.
To grow lots of leaves, yes you absolutely will need to fertilize and water it like mad once the pumpkin is growing! I'd prepare a really great bed with lots of compost and other good organic amendments for when you transplant them outside. Rich soil, tons of nutrients, and lots of water will grow you big vines, and big pumpkins.
As far as planting early, your weather is going to determine when you can put it outside to grow in the ground. Plan to start the seeds indoors a month or so before the last frost date. I used to live in Salt Lake City, and our last frost date came on average about May 1st. Yours might be later since you are further north. Also be prepared with a blanket or something "just in case" a late cold front comes through after you have planted your pumpkin plants out. You will need to cover them at night to protect them or else they could get nipped by frost and either die outright, or be set back by weeks, if they survive.
