It doesn't matter how the money was invested. Your basis in traditional IRAs does not apply to any particular investment, it applies to all of your traditional IRAs in aggregate. Assuming that you made no regular contributions to your traditional IRAs that you did not deduct, your basis in your IRAs is simply the amount of basis transferred from the 401(k). That amount would normally have been reported in box 5 of the Form 1099-R that reported the lump-sum distribution from the 401(k), which should agree with the basis amount shown on your 401(k) statement.
