USAC is not a corporation. Learn about MLPs before you post--you might be be surprised at the differences. Alerian has an excellent primer.
Slow down, you have at least until the second half of 2016, maybe longer depending on the approval process. Figure how much your tax will be (or consult a tax accountant) so you have a real number to work with; remember the gain will be long-term and you can spread it out over 2015 and 2016 by selling some in each year. Also, review the rest of your portfolio and clean out any losers--you can buy them back after the wash-sale rule expires if you really want them, but ITM, the losses will help offset your gain.
Since you're already invested in a natgas company that owns pipelines, consider buying midstream natgas mlp's or their GP's--most report losses for tax purposes that may offset some of your gain even though you'll receive cash distributions (you'll have to deal with K-1's, but doing that is usually cheaper than paying income taxes).
Last, take comfort that you have a GAIN! After the last few days, you are in the minority.
ETE reportedly transports 20% of our natgas, WPZ transports 30% and is a major factor in the Marcellus/Utica. Kelley will make it happen and the combination will be a juggernaut.
No. REITs must distribute 90% of TAXABLE, not book income but MLPs are not subject to such restrictions. In fact, most MLPs distribute more than their income--that's what DCF means.