(petitioner owned apartment building that school district acquired via eminent domain proceedings; petitioner did not replace property, thus gain not avoided under involuntary conversion rules; petitioner's purchase price of building $82,500, but IRS disagreed as to cost of improvements that petitioner made to building; petitioner failed to carry burden of prove as to cost of improvements, so petitioner's basis not as high as claimed by petition and taxable gain more than reported; petitioner bore burden of proof because of failure to substantiate basis and maintain required records).