Why do you want to cut it up? Cutting usually take much better and grow much faster when they are at least a foot long. I would plant the whole thing, then take cuttings in the future after it roots and gets established.

And yeah bridgesii like to throw some aerial roots. They're a little crazy like that.

Trichocereus species
If you ordered an unrooted cutting it should be planted with the growing tip upright. Middle sections have an arrow drawn on them and should be planted with this pointing up. Regular succulent soil enriched with a little organic fertilizer works great. Trichocereus are easily propagated by cuttings- cut sections that are at least 3-4″ long and place in a warm well ventilated spot for 3-4 weeks until a strong callous has formed over the cut surface, after this the cuttings are ready to plant. It is best to use a smaller pot to root your cuttings, then transplant once fully rooted. Cuttings can take a few weeks to a few months to root. Water irregularly until rooted. Once established they can be planted in large pots or growing beds. They really take off when given lots of horizontal root room. If using these plants for grafting, it is best to keep them in smaller pots which are easier to handle and wrap rubber bands around. Larger plants can take full sun when planted in the ground or growing beds, but it is best to give them a little shade otherwise. Many Trichocereus like lots of water and higher nitrogen fertilizer than most cacti. During summer you can really pump them like watermelons. They make great landscape specimens. The nocturnal flowering species bloom spring through fall, usually with the lunar cycles. The diurnal flowering ones bloom heavily late spring through summer. To produce the sweet edible fruit, 2 seperate clones or species are needed. To insure best pollination and fruit formation, hand pollinate with a paint brush. Nearly all species are somewhat cold hardy, even when wet. In general and to be safe keep them above 25° F.

agreed
they take forever to root and grow to mature girth if small cuts are made

How high up from the callous are these aerial roots?

I'd just plant it deep enough that it sits steady in the pot. I wouldn't worry about the aerial roots. They will either grow down into the soil, or they will eventually shrivel as new roots grow from below the soil.

They are not to fussy. Staying a half inch or so away from the roots is safest. Planting them so the roots are just touching the soil or barely buried is best.