duanedr 

Using a 38mm downtube on a standard BB shell will be tricky. For anything that isn't lugged, I use a T47 shell. The larger shell decreases overlap between tubes and if you offset toward the bottom of the shell, you can avoid it completely. Also, what headtube are you using? a standard 1-1/8" is 36-37mm which is smaller than your DT so the ears are going to require some work and delicate brazing - or you'll want to ovalize it.