I know this thread has been dead for the past year, but found it interesting and wanted to add a new twist to the request for a long block 380 BL motor.
With the established technology of advanced timing on BL motors, making a 380 motor the same length as a 540 would make for a light weight motor to replace silver can classes.
Consider this:
21.5 540 BL motor is rated 1,800kV at 85w
18.5 380 BL motor is rated 4,400kV at 82w
(both are listings from Novak site)
Assuming a higher torque 380 long block motor designed with an end bell that would allow mounting in a car designed for a 540 resulting in a higher power to weight ratio. Naturally, gearing would have to change to reflect the higher RPM 380. But overall, the weight would be reduced. I fear that in the end, it would just make for another class to dilute the existing classes.