Job work in 44 ad

Yes    

if i pay tax on 8% profit whether income tax department will not ask me anything except turnover

No. For Eligible assesse section 44AD can be adopted for eligible business.

As the method of caclulating tax is on presumption.......... no more requirement of proof other than turnover.

Read:  ''The scheme gives a great relief to the assessee in respect of maintenance of books of account. An assessee, who adopts the provisions of section 44AD, is not required to maintain books of account as per section 44AA (applicable only for business covered by this section). Further, in respect of such business, the provisions of section 44AB (relating to audit) are also not applicable. ''

You can get more details from  Tutorials/Section-44AD-Theory.