Please refer to Rule 2A of the Service Tax Valuation Rules, 2006 which governs the valuation in respect of Works Contract Service. Explanation 2 to the said Rule reads as follows:
Explanation 2.--For the removal of doubts, it is clarified that the provider of taxable service shall not take CENVAT credit of duties or cess paid on any inputs, used in or in relation to the said works contract, under the provisions ofCENVAT Credit Rules, 2004.]
Accordingly, you cannot avail the credit of excise duty paid on inputs. The logic is that material value is abated from the taxable value and when value of material is not included, the question of allowing credit thereon does not arise.
Also, please check - if it is original works, you should be paying tax on 40%, otherwise on 70% of the total value, in terms of the above cited rule. It is not clear as to how you are paying tax on 30% value.