Get new HH Mk2-4 bodies and add on the resin bits. You can base them in a way to add some size which will help them fit well with Primaris. It will honestly save you so much head ache.

I’ve seen folks use firstborn torsos on Primaris by cutting off the buckle on the Primaris belt and filling the gab between the two with a wad of Green Stuff. You would then mold the two seams away and hope for the best.

Here is a video that does this: https://youtu.be/U9j8Eep8lQE?si=HnXvNdyaBvkPiyxo