I have a 1976 it 400c with a cracked has tank. It goes all the way down the middle. Making it useless. I have been on the hunt for a gas tank for about 2 years. After posting here, checking ebay, and even reaching out to several people on instagram trying to source one I do not believe I will be able to find one. This leads me to wonder what other tanks will fit. The tank is really unique due to the frame. But the 76 YZ 400 is on the "same" frame from what I understand. So does anyone know if the metal tank from a 76 yz400 will fit the frame of a 76 it400? 

It does look like the same exact part number was used for the frame for several models...  MX and YZ 250 as well.  (See Partzilla for complete list of potential bikes with same frame. www.partzilla.com/product/yamaha/509-21110-05-33 )

Two other things that could cause issues are the triple clamps and the seat... top clamp seems the same.  Seat is different.  

If I remember correctly GFrench built an 1975 MX400b replica from a 1976 IT400.  MX400b is one of the bikes that share the frame part number and he used the MX400 tank on the bike.  It looked factory.  You might try sending him a PM to see if he had any issues with the seat or anything.

Note that the Partzilla list is not saying 77 and 78 YZ250 frames are the same, only the 76 YZ250c.

The parts diagram kinda sketches me out. It does not look like the same tank at all thats on the bike. www.partzilla.com/catalog/yamaha/motorcy...976/it400c/fuel-tank

That diagram looks like the 76 tank...  77 was completely different without the "pucks" like the metal tanks had.  Have you verified your VIN?

Or someone could have already modified the bike for a different tank.  Aftermarket probably made tanks as well, just not available anymore.

This tank fit my 77 it400   YZ 250/400 (78-79) IT 250/400 (78-80) XT/TT 500 (76-81) UNIVERSAL #SFLA | Clarke Racing
Its available in several colors but the it400 blue but is close.  DC Plastics   also has all the other  plastic needed for this bike including decals.