Thought it might be useful to share some pictures of my Kifaru Express which has been modified with some HPG components. The original pack was a very sturdy platform (1000d Cordura, double bottomed and #10 YKK zippers) but I found it to be lacking in terms of modularity (only one external pocket) and comfort when carried on the shoulders.
The HPG shoulder harness can be attached with some additional plastic hardware (webbing reducers and large tri-glide). I found some ITW Nexus QR buckles and attached them to the harness as well.
The Aston Pocket can be easily attached to the pack with the straps that are provided, but I had to use a pair of G-Hook ended webbing straps from my Ute to go over the lid (the admin pouch was sewn on by the previous owner).
The compression straps of the Express have been fed through the ladderlock and attached to the 4 SR buckles.
I have found the Aston Pocket to be the perfect 'beaver-tail' style attachment for this pack, but the lack of retention within the mesh section of the pocket was frustrating. I've added some mesh with Velcro backing to create a divider that prevents items from falling out. Items are still easily accessible as only the corners of the mesh have been sewn down.
A Tara insert also fits perfectly into the water bladder pouch in the pack of the pack. It's 21 inch stays make it reasonable for EDC and great for day/overnight hikes. I've also added a Wraptech belt onto it for greater load transfer to the hips. Another addition is a waterproof zipper (I am aware that the stitching used to attach it negates the effects, but what the hell) and corresponding internal mesh pocket on the underside of the lid.
I am slightly disappointed that these packs are not made anymore, but understand why. Demand for the 'tacticool' stuff from Kifaru dried up a while ago and the price point was pretty high for what you were getting.
Hope this helps anyone who looking to do something similar. I was lucky to pick up a G1 Express at a good price; they carry better than the G2 version as they do not have unnecessary padding in the shoulder and lumbar area.