Picked up a 77 dt400 for $50! Even have the title. Engine was disassembled in the early 90s for a rebuild, but never got put back together. 
im trying to find if anyone makes a full gasket kit of them. I’m pretty sure every thing needs to be gone through. Many parts from inside the right side case are missing. And the cases were already split. Does the dt400 share any of the clutch or kickstart parts with any other models? 

Greetings from Tennessee. 
   You can look up parts on partziila high lite a part you want and scroll down to the bottom of the page and it should show what other models parts fit your bike someone will be able to confirm if 250 parts interchange

According to partzilla everything is the same. I think something is messed up with that. No way a 250 and a 400 have the same piston and crank.

 from Salisbury, UK.

Try the same exercise, but using the CMSNL website (they are in Europe).

Always try and source genuine Yamaha parts first - they are out there. 

 

Yes, most of the Yamaha parts lists are shared among similar models... important information got lost when they got turned into digital parts list and put online.

You'll usually see 2 part numbers in the list for say a piston... one of the numbers is often at the beginning of the list... to tell which one of the two you need you need to check the prefix of the part number. Prefix is the model code and you can usually tell which part you need when there is a choice by the model code.

Keep in mind that MANY parts are identical between a 250 and 400... but certainly not pistons, rings, crank, cases, cylinder, head, etc.

If you can find an original parts book they clearly specify the differences. Or maybe CMSNL has done it? I don't check them often.

I’ve bought complete gasket sets from cruzinimage.net for around $25.
They work OK. Not as good as Yamaha gaskets, but they are a lot less money, and I try and play this game as cheap as possible.
I just checked and they have yours for $21.50.

www.cruzinimage.net/?s=DT400

I’ve even purchased a complete bottom end or engine from sellers just to have an example of how they go together. I did that for my DT400C, RT1B and even have a spare for the 68 DT1. Just watch for sellers that ship reasonably and don’t require “Freight “. I’m sure I haven’t paid over a couple hundred bucks for one and gets me a template for putting them back together plus any spare parts I might need.

Pedalcrazy is right I bought a complete AT2 engine from eBay everything but the carb had the oil pump, generator, kick and shift lever for $130 including shipping because I have a At2 and a AT1c so if I need any parts I have them.