Porters can have CG issues, particularly B2-H4 models since the gross weight keeps going up but the cabin doesn't get any bigger. Depending on your bulky cargo, you may or may not run out of room first. If it is bulky enough, you won't have CG issues.
Since I flew Porters, I have heard that Pilatus put out an AFM revision with the new planes that adds a cargo tie-down section. From what I was told you will be doing good to get 550 kg of cargo tied down in the plane legally. I have asked a couple current Porter pilots flying B2-H2s converted to B2-H4s about the new cargo tie-down section, but they had no clue what I was talking about. Not sure if it doesn't apply to all Porters, if Pilatus was not doing a good job notifying operators of the revision, or if that operator was a little lax on getting the AFM updated.