Due to shaft girth, it will be harder to line up the 3/8 on an extension - do able, but you would be better off with a slim T-handle allen, but with a ball like below:
Either way, get under the sump with a phone light and work out how to line up the tool.
Recommend:
Dry practice installing the sump without sealant.
Get all the bolts slightly threaded so that there is some movement in the sump, leave the 2 hidden bolts till last then tighten them all up fully.
The bolts have a really low torque value, don't go mad tightening them up.