The closest thing to arthropods that have colonized land is not cephalopods or molluscs. They are crustaceans.
And an awesomely land adapted crustacean is the pillbug, which is an isopod.
A Different Way to Breathe
“Like their ocean ancestors, pill bugs have gills,” said Wright. Gills
work great in the water. They’re basically exposed mucous membranes
that absorb oxygen out of the water and into the blood that feeds the
rest of the body. But on land, gills are a liability.
If the pill bug dries out, its gills won’t function properly and the
pill bug can suffocate. That’s why you usually only find them in damp
areas, like under a dead log. If they start to overheat and dry out,
pill bugs will even roll into a ball to protect the remaining moisture
on their gills.
Here is where being big helps. Pillbugs need to keep gills moist, so they stay where it is moist. We need to keep our lungs moist too but we truck around great reservoirs of moisture in the form of blood, with addtional water produced on demand by the oxidation of fat (CHx + O2 -> CO2 + H2O).
Your giants use gills to maximize surface area. Because they are giants with large volumes they are full of water - or better, fat or oil because it is lighter. They can keep their gills wet because of these onboard supplies. Their exoskeletons are thin or vestigial and I here assert that the reasons isopods have not displaced insects on land is because the carapace is difficult for them to evolve around and it gets too heavy and bulky as it gets larger. I was interested to read that one of the largest truly terrestrial (not littoral) isopods (Porcellio magnificus) prefers low humidity - which it can get away with because of its size and onboard water supply.
from comment: /Interesting, but how could this evolve in other groups of arthropods?/
You could start with insects that had already evolved gills. Behold the hellgrammite!
The hellgrammite is the large predatory larva of the dobsonfly. They have external gills to facilitate water breathing.
Just as the ancestors of whales left the water, got adapted to the air, then brought those adaptations back to rule the waters you could have an insect adapted to air, move to the water, then back to the air with its respiratory adaptations. I like the idea of giant hellgrammite as a crocodile-like ambush predator.
The molluscs have done things like this - terrestrial snails that go back to freshwater and evolve secondary gills.