Mikuni 36RS on Z1R - stock airbox?

My 1978 Z1R was a basket case when I got it did not come with the stock airbox, and has mikuni 36RS flatslides - big bore kit and head work, it runs great now. The previous owner located the stock air box and sent my way, and I'd like to go with more of a correct look for the bike. What I am reading is the mikuni and the stock are both 42mm, will the airbox fit? I have a 4 in 1 header on it now, but also have the stock exhaust to put on. 

If I swap those over, the carbs and handlebar controls are all that is "wrong" with the bike. 

Fitting the airbox to that engine with those mods will likely restrict it's performance and you will have to re -jet too.
For more original looks you may be able to just fit the visible part of the airbox with no internals etc to hopefully keep the maximum airflow but there's a choice to be made between going stock or keeping the extra power.
Depending on the state of tune, fitting the stock exhaust too will certainly substantially restrict any advantage the carbs, bigbore and headwork gave.