Tailoring/Modifying

DigDug, Yeah, I think it would work, however, there are a couple of pitfalls I can think of:

1. i'd avoid too large of a dart because you'd end up with a lot bigger panel on the front than on the back if you get to aggressive with it.

2. you'd need to remove the entire scoop hem first, then dart, then redo the scoop hem, otherwise, it would look like crap.

3. place the darts in a manner so that they look like the are part of the shirt design.

I recently redid the front hem/scoop of a j.crew flannel workshirt that was comically long in the front (the front of the shirt was 1.5" LONGER than the back); it was surprisingly easy and looks perfect.

The key is to work slowly and iron out your folds carefully and precisely.

If I were you, I'd iron some darts in, pin them in place, and wear around the house first to see if you get the change you are looking for.

I also have a question about tailoring... If you drop $100 or $200+ on one piece, is there a highly recommended method for hemming or tailoring, or would dropping it off at a local tailor do? I've heard of people sending their stuff to Railcar to have them do the job, which seems understandable if the difference is great enough. Thoughts?

I always warm Soak my jeans before getting hemmed. They could shrink giving you high waters.
If your double cuffing, it doesn't matter (IMO) how you get them hemmed.
If your not or going to wear a single cuff, a lot will depend on how True you want to be to Raw Denim.
For many only a Chain Stitch cuff is exceptable. If that's the case you will need to send them out to be done (RailCar, AB Fits).
If not then most any tailor can give you a lock stitch (I think that's what's it's called)

Yes, RailCar or ABFits. If you were to bring them to a local tailor, they would do it like a normal pair of pants. They would cut the ID off.
It's a pain and another expense to have it done right. But if you don't you'll end up with a pair of screwed up jeans that you paid good $ money for that you Never will wear. I have 3 pairs like that.

I just scored this used Flat Head wabash vest off eBay.. It's a size 40, and if you look at the photos, the previous owner took out the pleats in the back. I'm assuming this won't affect the integrity of the vest, really, but maybe just not have as slim of a fit. What do y'all think?
I'm assuming that, if I wanted to, I could have the pleats put back in, even following the old faint lines... maybe even send it to Railcar to have them do it.

www.ebay.com/itm/The-Flat-Head-Made-in-Japan-Wabash-Denim-Vest-Waistcoat-Size-40-/131697481751?hash=item1ea9c81817:g:3hEAAOSwKtlWklil

pleated raw selvedge denim - a horrible trend coming 2021

For me Yes.
I didn't for my RailCars (16oz Cone). After hemming and a few more wears I washed (tub, warm water) and they ended up shrinking an inch. I still like them and wear frequently but they are a little high water, so I just make sure I'm wearing nice boots.

Agree about washing/soaking before hemming. I've always cold soaked my jeans and then hang dried but from now on I'm gonna do a hot soak/hang dry before hemming. It's weird because I thought I had gotten most of the shrinkage out with a cold soak on my G American Standards but when I gave them their first cold machine wash I swear they shrunk in the inseam a little more. It's not noticeable to the eye but I'm used to the way my cuffs drape over my shoes.

I forgot to address your actual question... I don't have a single pair of raw denim that hasn't lost at least an inch post-wash. Pretty much every one is a little different, so it's definitely a good idea to run them through whatever your future cleaning routine will be at least once before you permanently alter them.

I should add that I do make certain exceptions when it comes to soaking jeans before hemming. My 3sixteen's are both slim fit. My st-100x are snug as it is so if I did a soak and then hem I'm certain I won't be able to get them on. So those will likely get no soak and just a hem, unless I double cuff. Same with my st-140x. My 3 other pairs of Gustin's would probably be safe with a hot soak since those have bigger thighs

Well, 2 options: start eating (or drinking) a lot. 

Honestly, can they take a jean down 3 sizes? It would essentially be a complete disassemble and reassemble to do that. And given the weight, you may be paying a lot from AB Fits...(assuming they take the order because of the weight). 

The alternative could be inquiring with Manny (Indigogene) to quote the work. 

Ok, I'll try to talk you out of it.

It is an absolutely terrible idea. It will cost a lot and probably not fit correctly. You are not talking about a hem or even a leg taper. You are talking about pulling in the waist 3 sizes. This is asking for disaster. All ratios will be wrong for the rest of the jeans. The entire jeans would have to be reworked. 

You are probably over excited because you think the jeans will never be reproduced. There is always a new heavyweight offering from every vendor out there - nearly ever year or so. Look at Gustin, Iron Heart, Samurai, etc. for the last 10 years. New 23 oz + offering in new fits every year. If denim makers stopped new product offerings, the whole retail system would collapse. 

Wait and buy something that fits you correctly. 

(What is this mystery never-to-be-made again pair of jeans?)

Edit: Also fewer and fewer companies will even do the modifications. E.g. You mentioned railcar - they won't modify 25 oz any more. 

Edit: I personally have done less invasive modifications at least twice and regretted the outcome and cost both times.