I purchased a 1978 DT175 to replace my 1982 Honda XL 185S.
I have done a little bit of work so far and completed a 12v conversion. Im curious will a 78 DT250 tank fit my bike and are these bike like hondas where I can put any engine frame combinations I want together, my experience in Hondas is that the frames are modular from 125-250 then they get bigger for 300-500.

Greetings from Tennessee 

1978 125 and 175 are virtually the same except for displacement and cosmetics.

1978 250 and 400 are similar to each other, not the same as the 125/175.  250 tank will not fit a 175, not even close. 

If you are in the US, 1978 DT125/175 tank is "one year only"... Major "tell" is conventional front forks in 78, US models got leading axle forks in 1979 along with a new tank and frame design.  (tank mounts were changed)   Make certain you have a 78 model by the VIN, not by an unreliable method like the paperwork or production date on the steering head.  Prefix should be 2A7 if I remember correctly.  VIN tables and manuals are in the Tech Library.

 

That's a lot of info I was looking for, thank you. I will give the tech library a look and I might end up selling this DT175 to get back to an XL with a title this time.

Even though it's a decade older, 78 DT175 is much more fun and capable on the trails than a 1980's Honda 185 or even 250 in my experience. The Hondas are not bad bikes though.

What did your "12 volt conversion" consist of? Swap to a VAPE or something else?

I agree with MarkT the Yamaha is just a better trail bike faster and handles better 

Had my time with 185/200 Hondas. Megacycle cam, 12.5-1 wiseco piston, big carb & everything. Had monumental blowups with valves in piston & even conrod out the front & flattened the exhaust pipe haha. In between blowups it went like hell--sounded good too. Had mine in a PE250 frame for vmx.

I have a 1980 DT175 with the rear mono-shock, 6-speed, and that thing is simply amazing. Low center of gravity, easy to ride, and very capable on rough terrain. It was a junker I bought with no air filter and fried reeds. Cleaned it up, pulled the oiler and added Boysen reeds, use premix due to all the missing parts, and she took me at least another 2-3k miles of reliable punishment in the Arizona desert. It'll do 65 MPH on the street. Now the piston is making pinging noises and its lost power. Time for a refresh on the motor at 12k miles. I think I want to go for a naked Ducati look on her with a Yamaha blue frame of course and probably get the autolube back in place. I always wanted an CL450, SL350, XL250, or XL350 Honda, but haven't come across the right one. I have an SL100 that is a solid bike, but it is nowhere near as good on the trail handling bumps.

Not sure if you've had your DT175 running to ride and compare to the SL/XL Hondas, but you will find Yamahas much faster and more capable than a Honda. The 2 stroke engines are not as complex and way lighter with a better character for their intended use.