The 389 carb is the bigger brother of the AMAL Monobloc 376, so not easy.. The smaller 376 will fit, but again it's a tight squeeze, here the top cable adjusters need to be substituted for plain collars as mentioned by RD to avoid the top frame tube. The plunger air filter connection has to be modified or simply discarded. Monobloc throttle adjuster screws can seize in the body, and shear off when you come to adjust the tickover.
Best to stick with the earlier 276 remote float type for an easy life.
Swarfy.