Damn man. You've got to be in bad shape.
I'm not the most familiar with methadone, quite honestly, but I've kicked from pretty much every other opiod out there at some point including some pretty large habits.
I think you can do some quick things to feel a good bit better in a short amount of time. I wouldn't worry about 'extending your withdrawal' at this point...anything you do to counteract your symptoms is going to be milder and way shorter acting then 'done.
I would ask the VA for a script for gabapentin. Say you're in horrible pain, and it's helped in the past. Most doctors hand it out like candy, and while it's not going to totally relieve your symptoms from a 'done jump off that big, it will help. To put things into perspective, 2000-2400mg of gabapentin daily relieved 90% of symptoms from a 300mg/day OC habit for me, and quite a few smaller jump offs over the years. I know that 300mg of OC is maybe...1/3?...the habit of 100mg of 'done, but it should still help.
I'd also order some strong kratom powder (most vendors will do overnight/weekend shipping), and hit the local pharmacy and stock up on generic loperamide and good old tylenol.
Once you have your arsenal in place, I'd dose this every four hours:
-600mg of gabapentin
-6mg of loperamide
-7.5-10 grams of kratom powder
-325 of tylenol
I really think that this combo will help, a lot, without getting you back on 'hard' opiods. You're still going to be dopesick, but I really think it'll help. Give the gabapentin a few hours to work after the first dose, too. I usually don't feel relief until 2-3 hours in. It's onset of action is REALLY slow.
I would avoid the fentanyl - it's going to cause more problems in the long run. The ridiculously short half life means you'll be dosing non-stop, too.