I have absolutely no experience with metallurgy, welding, etc so this question might be incredibly stupid. If so, pardon my ignorance.

I'm making a custom firearm that uses many off-the-shelf components including an AR10 barrel/barrel extension. The problem is, the way the bolt carrier is designed doesn't allow for a standard AR10 bolt. I need to rotate the cam slot by 90 degrees and move it back a smidge so it doesn't interfere with the extractor.

At first I was hoping to find a manufacturing who would sell me unfinished AR10 bolts (every operation except machining the cam pin hole), but so far I've had no luck on that end. I've been racking my brain trying to think of other solutions and thought of filling the existing hole with weld then machining the cam pin hole where it needs to be. This obviously would destroy the heat treatment.

Would I be able to get the bolt re-heat-treated while keeping dimensions within operational margins, or would the process screw up things like headspace?

I've put a lot of effort into this firearm, and it's going to kill me if I hit a dead end because of the bolt.

Did you try to combine Armalite AR-10 parts with DPMS parts ?
I would strongly advice against the idea to modify the bolt or the bolt carrier...

You might be better off machining your own bolt based on the AR-10 bolt. Could you change the design of your bolt carrier instead?

Unfortunately not. The more and more research I do on this topic, the more I'm convinced a custom machined bolt is probably going to be my answer. :/

Machining and treating the kind of steel used for a bolt is a bit above what my equipment can handle. I'll have to find somebody who accepts that kind of work.

Welding that big a hole on that small an object is probably going to cause warping of the bolt.
It's possible it could be re-heat treated by a professional service, but that too can cause warping.

About all I can suggest is continuing to find a manufacturer who will sell a "blank" bolt or have a better CNC machining service make a bot from bar stock.
Won't be cheap but it's quite doable if you have blue prints or a finished bolt they can redesign for your changes and manufacture.

AR-10 are not standardized across the industry like AR-15's. There are several companies making "AR-10's" and almost none of their parts can be interchanged. Trigger parts will work. Once you get into bolts, bolt carriers and barrels everything becomes proprietary.

You are talking the long way and the wrong way around the barn. Buy the compatible parts you need and call it a day. Even if you have to wait until panic buying subsides you will be better off buying the correct parts then modifying something that isn't.

Since you are basically on your own building an AR-10 at home, the best way (IMO) to address your clearance problem would be to modify the upper and not the bolt or bolt carrier group.

Making changes to the upper will allow you to use unmodified OEM supply lines. The hard work, heat treatment, precision machining have all been worked out and replacement parts will easily swap in.  

Not knowing what your exact problem is, I imagine you have serious clearance problems trying to mill, EDM or file clearance into your existing upper because of a tight work space?

If your home built upper doesn't allow the cam pin to rotate you could mill an oval area all the way through the upper where there is interference to provide that clearance. You can weld (if steel) or drill and tap to attach a humped cover plate that hides the hole thereafter.

I made an AR16 (AR18) style 308 rifle and I used a SCAR-17 bolt and barrel extension, because I wanted the bolt cam pin hole at a different angle from an AR10 bolt and I used a cubical bolt carrier.

I thought about trying to machine the bolt carrier for the AR10 cam pin location and angle, but SCAR-17 parts were available.