Not calling this a restoration, as I'm not doing any repaint, etc.  Pretty much reassembling it with existing parts from two bikes, one will end up as street legal (have a title), the other a vintage MX (engine work already started by the previous owner, and most of the parts supplied).  Making some progress with the street legal version:

 
This is from the weekend
 
After last night

I'm finding some interesting differences between the two, wondering if the second one is a 76?  The titled 400 is a 75, and it has different triple clamp hardware (chrome, 13mm heads), the speedo/tach mount is steel vs. formed plastic on the donor bike, and the oil tank says ENDURO 400.    The donor bike oil tank is labeled ENDURO. The head light mounting bracket seems to be the same, but the lower reflector mounts and rubber washers are different between the two. The titled bike had a spray bomb restore at some point, the other is more beat up but has less miles and seems mostly original.  The titled bike has the date of mfg rubbed off the label, the donor bike shows a mfg date of 10-75.

Kind of interesting. Wondering if these are running production changes, or they are different year models?  

1975 said "ENDURO 400" on heat shield and oil tank. 

1976 only said "Enduro"

Fuel tank graphics are slightly different as well.

Check the VIN numbers against the chart in the Tech Library to identify what you have.

You'll want to use the 1976 harness with the voltage regulator if you can.  1976 was the first year for that unless someone added a voltage regulator to the 75. 

Great info, thanks Mark. I was not planning on a harness change, but will investigate. I do see the voltage regulator on the donor bike, but haven't assembled the titled bike that far as of yet. I got that one disassembled.

DEET has the most knowledge about exact months but generally production ended very early in the year with the vast majority of a model year produced in the previous calendar year. 

If you check the VIN, the bike with the 10-75 date is almost certainly a 1976 model.  Production of 1976 models usually began in early 1975...  (maybe as early as March 1975?)... and ended in early 1976.

 

Still making progress. Have a new gasket on the clutch side cover, and put the oil pump from the engine that had it operational on the stock engine for the titled bike. Engine is back in the frame, will be buttoning up details now.

Kind of interesting working on an old bike; the aluminum and plastic tie wraps to hold wiring, formed metal bits, etc. So much plastic on the newer bikes! Trying to keep it pretty much original, so no zip ties allowed. Since the one being assembled was apart when I got it, the spare that was mostly complete and fairly unmolested is providing a good standard for some of the details.

Several differences between the 75 and 76 however, even motor mounting was changed for 76.

Should get it mostly put together by the weekend, depending on what I find that is missing or needs additional repairs.

Chuckw - love to see some pictures of your ride, I am basically restoring a "$600 bargain :-) " '78 DT400 parts bin extravaganza

if your having an issue finding the small bits on eBay, I appologize!  Two items elude me, the keyed air cleaner cover and: BRACKET, REFLECT 11M1-85132-62-0 that attaches to the tail light bracket and sits behind the plate and accepts a cool little reflector at the bottom,  which I really want to findwaiting on proper throttle cable (wrong of course) and almost ready to paint the tank.  I had targeted Thanksgiving, but I need another week or two to hopefully get the tank done without any errors.  
 

Thanks M, the '78 sounds like a fun project, I'd like to get a DT of that generation eventually.  Mine started as one complete bike in poor shape, and another that was decent but disassembled and partially turned into an MX bike.  Including some extensive engine mods.  I've been fortunate not to need a lot parts, having two bikes and with the extra's from the former owner, but will be looking for a stock expansion chamber/pipe soon.  Here are some pics, as purchased and current. DT of that generation eventually.  Mine started as one complete bike in poor shape, and another that was decent but disassembled and partially turned into an MX bike.  Including some extensive engine mods.  I've been fortunate not to need a lot parts, having two bikes and with the extra's from the former owner, but will be looking for a stock pipe soon.  Here are some pics, as purchased and current.  The one in the truck is the one going together now.
 
 
 

Mostly done, waiting for a rear sprocket still, grips and the plastic side covers to get cleaned up and installed. Had it running today, have some wiring to sort out and will test everything once the battery arrives.  Started after just a few kicks!  Thanks for the blog suggestion on the AGM replacement, have that coming.  Other than the rear signal's I think it has everything?  

Will need a new pipe eventually, this one had some pretty crude welding done on it , apparently it had some cracks around the upswept section.

Between the two bikes, I have some vintage grips!  Yellow ones are ODI, and are labeled for install on an ATC.  Black ones are the strange shaped Oakley's, supposed to fit your hand by being longer towards the front.  Pretty sure I had some back in the day and they were not good.   

Awesome job, looks great!