I would be surprised to hear that the Judiciary Committee had recommended to the House an article of impeachment regarding a tax return that could not be proved.
Also, are you sure about the statute of limitations? There may be a time limit to challenging the accuracy of a tax return for purposes of amending the tax due. This is not necessarily the same as a statute of limitation concerning commission of a felony by use of a forged document. Perhaps you can clarify what you are suggesting and also suggest a reason that Nixon paid the tax.
I was working for the IRS at the time it happened. There is a 3 year civil statute barring additional assessments unless 25% or more of your income was omitted from the return, then it is a 6 year statute. There is no statute of limitations if there is fraud. Since the issue involved a deduction from income, not the omission of income the 6 year statute did not apply. The fraud aspect of the back dated document was never pursued by the IRS, otherwise he wouldn't have needed to voluntarily waive his rights to civilly pay the tax.
My guess is the IRS couldn't prove Nixon knew the appraisal was back dated.