How much work to install 836cc kit on 75 CB750?

Will it just plug and play or will I have to get it bored out?

836 is probably just a bore and hone, but may need the liners to be removed and bored to take larger liners which then have to be bored to size after they are fitted.

You can use the stock sleeves for the 836 kit.
But that is about as large as you can go without removing and adding new sleeves.
I had to put in new sleeves when I did my 890 kit on my 1981 CB750K.

Just about any local machine shop that does head work can do the machining.
Just give them a call or I am sure there are several on the list here that can handle it.

Good luck!

Should cost $50 or so per hole ($200) to have the cylinders bored and honed - make sure you have the kit on hand before you do it - a GOOD machine shop will measure each piston and bore the cylinder to match that specific piston. There are slight differences from one piston to the next.

Sure, this isn't critical, but it's a sign the shop knows what they're doing. Most auto machine shops can't bore cylinders that small either - find a shop that does motorcycles. And have your valve seats ground etc. at the same time, guides checked, new valve stem seals...