The option to use bus duct assumes that the PoCo hasn't set the transformer. Indeed, I'm assuming that the substructures haven't been constructed. In which case you'd engineer Service to use standard components.
PG&E has cut sheets for laying out bus duct transformers. Not uncommonly, the transformer is situated right next to the building (concrete curtain wall) such as to permit getting from outside to inside with standard elements.
If the PoCo has set it's transformer and you'd need 40 feet of bus duct then expect them to shoot it down. They do not favor extended runs above ground on the secondary side, and the secondaries are deeded to them as a matter of course after acceptance.
At that distance, I'd say copper in Schedule 40 PVC, perhaps some DB120 for the deep horizontal raceway, is your only game. There is no choice.
The PoCo is going to call this tune.
