Only pre-tax money in your traditional IRAs is permitted to be rolled over to the 401(k), so the rollover of your SEP IRA (a type of traditional IRA) to the 401(k) moved only pre-tax money and left the after-tax, nondeductible traditional IRA contributions in your traditional IRAs. If you made no other traditional IRA distributions beyond those mentioned here, there is no need to know the year-end balances for prior years.
To calculate the taxable amount of your Roth conversion Form 8606 for the year of the conversion needs to know your basis in nondeductible traditional IRA contributions (from your the Form 8606 filed for the most recent year prior to the year of the Roth conversion, the amount of any new nondeductible traditional IRA contributions for the year of the Roth conversion, the amount of the Roth conversion, and the balance in traditional IRAs at the end of the year of the Roth conversion.
Presumably your current balance in traditional IRAs is greater than your basis in nondeductible traditional IRA contributions, otherwise you would not have asked the original question. Is there some reason that you do not want to roll more from your traditional IRAs over to your 401(k) so that you'll only have your basis in nondeductible traditional IRA contributions remaining to convert to Roth nontaxably, rather than having the conversion be partly taxable and partly nontaxable?
