NEAT! I have a 76 DT250 myself. I'm curious to see how that sister model MX250 pipe you have works out. I have a pipe from a 73-74 MX250 which looks alot like the stocker but lighter and more of a chamber.. however never got it fitted. I also hav a hand-made pipe that two of us on Yamahaenduro had made, but it totally killed the low end, and I have never been able to make it work really good up top, but it wants to rev to the moon.. The consensus was those guys didn't know what they were doing.
I'm anxious to see this develop and see how it runs for you! What carb are you planning on using?