I was recently at a tournament, the guy had a unit of savage orcs with big stabbas.

The question is: Does that model must be in base contact with the unit or the get impact hits no matter what?

Yep has to be in Base to base with your oponent.

huh, where does it state that? As far as I know its the unit that has to be in base contact, not the particular model.

No, that model doesn't need base to base contact. The stabba rule just says to pick a model in the front rank that will receive the impact hits, nothing about picking one in base to base contact.

Sorry should have said I play in a WYSIWYG area and so that is why placing your Stabba is important. This should be FAQed before there are arguements about it :D as I can see it both ways. The way we have been playing it is like a single model unit upgrade ie a champion. That was my opinion and not fact.

Related to this, do big stabbas benefit from the choppa rule?

yes they do. I think it's in the FAQ.

There is no specific stabba model any more than there is a specific netter model in a NG unit. You could model the big stabba on a filler base in the rear ranks and it would still count.

I'm modelling at least two in a big unit because I think it makes it clearer to the opponent that it is a unit carrying big stabbas, not a single model carrying a special weapon.

It's pretty clear;

"Nominate a single model in the front rank of the unit to be the one that inflicts the impact hits."

No problem, you can nominate ANY model in the front rank. If they go wysiwyg (which normally I agree with, but the rules are clear here) point out that the big stabba is "represented by including one or more big stabba models in the unit."

You can have as many as you want, which might look mighty cool :D

RoV

To beat the dead horse, the Big Stabbas rules clearly state that a unit of SOs that includes Big Stabbas has the Impact Hits special rule.