Wires wear from vibration, and relative motion with the surfaces they contact. If you've got a problem area that stuff is awesome. For preventative maintenance, you've got a system that when kept well will last how many years? That's hard to improve upon, so IMHO your efforts are far better spent in the detail to follow exact routings, use snug fitting clamps, ties where appropriate... Then there will be no problems. You'll find that your OE harness will outlast the best quality split loom you could hope to buy. Besides, then you're going to have to get all brand new clamps anyhow, otherwise you've defeated the purpose by trimming away the loom right where you need it. So all of that effort towards the original will have to be covered either way.
Now if you want it anyhow... The original taped harness look doesn't work for you, you've got modifications that need different protection than the original... Most parts stores have it by the foot, I'm not blown away by those prices. To do a good, effective job with it, the wire in the loom has to fill 70 percent of more of the loom (the fuller the better, less vibration that way), so you're going to need a bunch of sizes. Pending an investigation into pricing (shop around and make sure they're quoting bulk), I suspect you'd probably be better off to pick it up locally.
The factory taped harness in these trucks, the tape will effectively eliminate the chaffing issues inside that loom in most cases. Loose wires with poorly filled loom (lots of movement) is where that shows up. With care and attention to detail (Tons of both...) that can make a very, very clean looking installation. If you get the opportunity, inch by inch untape some factory looms in a boneyard... See how they tape "Y" breakouts, "T" breakouts, "X" breakouts, "+" breakouts, mid wire starting/stopping of the loom, stopping at connectors, etc. You might also notice that a few sections are taped within it, and a ton of sections are "spiral wrapped" in tape, but not fully wrapped. You've got that part covered with the original, but it goes to the chaffing issue... The loom is the easy part, the separation between a good, effective, protective, and clean looking application, and a quick coverup patch job... That's the details... The tape, the clamps, the routing, the care to make sure it's routed and twisted so the split stays at least closed if not out sight.
