Are you converting from automatic hubs or does the truck have an automatic diff disconnect (ADD)? I'm 99% sure the ADD trucks have an important clip inside, so all you have to do is remove the drive plate, pop the 6 cone washers, remove the bolt and body and go backwards with the manual hubs. I'm not sure if the auto hub trucks have the same parts (these are the hubs that lock themselves and you back up to unlock).
On the manual hubs (and ADD) that bolt isn't strictly critical and AFAIK it's there for a couple of reasons.
One is it serves as a backup to the clip that is actually holding everything snug, which if that snap clip was to break would let the axle walk backwards out of the hub/spindle. So you should have that bolt in place for just in case.
Second, I always used to remove that bolt so that I could that location to fasten a cheater lever to pull on the axle. You will argue, cuss, throw wrenches trying to get the axle pulled in far enough to get that damned snap in place on the slot. So using a little bolt with a bracket so you can pull with a bar or something lets you have a free hand to get the clip on.
You won't see it with the hub body still on. BTW, I don't think this is an iFS hub, but the view is representative. On your hubs the brake rotor is bolted on from the back, so you won't see those bolts.
But when you remove the 6 cone washers and nuts, then pull off the hub body you'll see (or should see) the ring at the end of the axle. If you look close, follow the axle end splines back towards the wheel bearing, there's a small diameter clip in front of a thrust washer.
If the bolt breaks it's not the end of the world, more of an irritation. You can still put the hub back together but it's just a PITA. If the clip breaks then you're hosed. That is really the critical part.