I am buying a 390 with a 27.7 gp boom and 14' stick, 36" pads. I will be digging in very soft soil. I'm trying to get the best bucket. I'd like big as it will handle efficiently. I can get an 87" 8.0cyrd or 79" 7.0cyrd. I have talked to a few people who said an 8.0 yard bucket is no problem. Do you guys have any experience with this? I'm open to ideas.
A quick look at the specalog shows that your configuration recommends a 6 yard bucket. An 8 yard bucket 100% filled with, let's say 75 Lb/cubic ft soil at full radius would put you well into tipping territory. If you could get a very light duty bucket it may be feasible.
I'd say it would all depend on the empty weight of the bucket you are thinking about using, in the "ready to work" condition, compared to the weight of the recommended 6cu.yd. "standard" bucket.