Hi Raman,
If the client doesn't want tacks, depending on the size of your components, clamps and fixtures is one way to hold the parts for fit-up.  May be a little difficult to weld, again dependent on size.

Tyrone

Couple issues or problems here.
You might wish to ask your idiot client exactly how it is they propose to fit these assemblies.
Second, the reason we tack instead of fixturing as was appropriately suggested is that its faster and cheaper. Was the project priced accordingly if the client is going to be difficult.

I'm with Tyrone and JS here, there are inherent problems with this request if we have understood YOU correctly and if you have understood their request correctly. 

Also, per JS's comment, tacking is allowed and given guidelines for within most codes.  It is an industry standard procedure.  Thus, unless this was information made well known upon the bid process it is a change that can have a change order issued with cost estimates for the additional time in labor and then billed as an additional charge at the end of the job.

Bring that up, if it is the case, and I will bet you will be allowed to tack as normal.

Brent

The client is always right. Of course being right can sometimes come with added cost. No tacks no problem... for a nominal fee of course.

Just for the sake of argument here, what code are you working to?  What process are you using?  What type, material grade, etc is involved? 

Client specs called out originally or after you were awarded the job? 

Brent

if it was me
I would know from the bid and contract
what is needed and called for
the client sent out specs for the bid
then accepted the bid
and both parties signed a contract
(should be looked over and same as the bid papers)
a change from bid to contract should be taken into account and change the price
(but if a party missed the change...oh well...stuck and eat the loss, fulfill the contract)
a change from the contract voids the contract
and must be dealt with as new contract status
meaning new pricing
just my thoughts
as far as tacks go,
weld to 1/2" of the tack and cut or grind it out the tacks
sincerely,
Kent