whats interchangable yz400 and yz426 motor?

Hello all this is my first post in TT! Ive always looked and the foru m but havent got around to signing up till now. I have a prob here I just bought a supposed motor from a guy that was suppose to be a yz426 but long story short I get it and its a yz400 but he swears it has a 426 piston and head on it but obviously 400 crank. Not only that but case was damaged during shipment so now I need to buy a new case and its not what I paid for :/ But it does have a hot cam that I want to put in my 426 to make starting easier can I swap it out?! Better question to ask would be what is not interchangable between years. Another thing how would I tell which carb come with it. It looks identicale to my 426 but no accelerator pump it is there any difference between the 2?

I just rebuilt my '99 400. I used a 426 top end. Everything from the piston up.All works great. The carb is the same on the two also. The 400/426 carb is a standard FCR and has an accelerator pump. Hope this helps.

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