Man,
Height: We didn't have to cut the stacks - just use the shorter ones that are available. Jeez - I don't remember whether that engine was a 302 or 351... Your engine may not be at the same height as ours.
Cost: If you can find a set, typically $2000-$3500 for the carbs and manifold. But that's just the beginning.
Easy? Depends on how handy you are.
It is likely the carbs will have to be rejetted. If you pay someone to do it, it will cost roughly $1500. If you do it yourself without too many screw-ups, several hundred $ in jets and lots of patience.
Synchronizing the carbs? The least of your problems if you have the right tool and some knowledge of the process.
You will potentially get about 5-10% horsepower increase - but only if it's done right. And sometimes that ideal point is illusive.