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The 292 has a stock bore of 3.75 inches. To get to 302 cubic inches you'd need to bore it to nearly 3.82 inches. The difference is roughly .070 or 70 thousandths of an inch. Doesn't sound like much but you would probably need to get the block sonic checked to determine the amount wall thicknesses throughout each bore and then you'll be able to tell if there is enough meat to go to .070 overbore. The information at the link below seems to say it may be possible to bore as much as .125. Good conversation to have with your machinist probably.
For '61 and older blocks, .060 or .70 over is usually not a problem. For '62 or newer, it might be a problem because they have thinner walls. In either case, it is always smart to sonic check the cylinders.
Don't be too concerned with an extra 10ci. There won't be much of a benefit doing that. Go only as big as necessary. Regarding pistons, any "off the shelf" piston is only going to be good for a more or less stock build. If a bigger cam is also a part of the plan, then you want domed pistons, which will most likely have to be custom so while you're at it, use a modern ring package and light weight design to help it reach it's potential.
For what it's worth, 4.00" bore Ys have been built. There are very few blocks that will do that and they are early 272 blocks and at that point, they are strictly a street engine and primarily for a 4.00" bore bragging right since the cylinder wall is too thin for any real performance at that point. This should not be an expected outcome and would be mostly unrealistic, just for info.
The .125 overbore has been done but a sonic check is definitely advised for that.
Difficulty is easy, assuming you can build just about any other engine.
Probably $3000 to $4000 for a stock build done right. Could be more if you have it built. Could be less depending on exactly what is needed and what you want out of it. Up to about 300hp won't necessarily cost much, if any, more than a stock build. It's when you get over 300hp that it starts getting expensive more quickly because that is the point at which additional specialized work, specialized parts, or custom parts start to be required.