Alien Pack/LW2 weapon archetypes issue

[NotSoLoneWolf]

So, I'm trying to make a different SPARK class that uses a missile launcher instead of a BIT. I've got all the code working, but I have one problem

I have two different types of missile launchers, the micromissiles that the standard ADVENT MEC uses, and a new weapon, macromissiles, which have squadsight range, less area of effect, less ammo, and more damage. For the macro-launcher's model, I wanted to use the Alien Pack's MEC, the green/blue (Archer/Longbow) one with the cannon that has the gigantic barrel.

However, while I can add the archetype in the code perfectly fine, the actual archetype itself appears to just be the standard MEC launcher, but green. Taking a look in the editor at the .upk files, it seems the Archer/Longbow's extended barrel is a separate model, not at all attached to the archetype.

Is there any way I can "join" the two models into one archetype? The editor is very hard to use, and all the models I drag from the content browser to the creation space thing are completely black and almost invisible?

TL;DR how do I use the XCOM2/Unreal Editor?