A couple years ago, I found a thread on fitting a late 90's-early 2000's aluminum skidplate from a Honda XR, onto the 74-76 DT250-260-400. All it required was a little trimming at the rear for the drain plug on the DT. 
 I had also seen a couple threads on aftermarket higher capacity plastic fuel tanks for the same year DT's, but I can't find anything on either subject now! I'm not sure if google changed search parameters or what, but if you have any info on either subject it'd be much appreciated!

Here is one of the threads on tanks...  you might contact DEET and see if he has one. yamaha-enduros.com/index.php/forums/1972...fla-fuel-tank#132426

Most people just use the stock skidplate...  I remember someone making one from Kydex...  I don't remember seeing anything about a Honda plate modified but someone else might.

I stumbled on to an alloy one from an OSSA Pioneer that had already been extended by adding a section from another one. It actually fits quite well on the DT360A. I used the two top 6mm mounts on the frame near the engine mounts, and got two rear mount clips for an early DT1 for the rear. You wouldn't really need the extension.

I know there are OSSA guys around (I know one), maybe you could pick up something like that and make it work for you. There is also a seller on eBay selling an old one for a DT1 currently.