I can give you a rough idea of whats involved so you gain a full appreciation of the difficulty and skill involved with doing this. That first sentence translates into real money to have this done propperly and rightfully so.
You ever flared the end of a copper pipe for a fitting? Thats swagging in its simplest form. Copper, being very malleable, will readily swag. A pistol frame, being made from heat treated alloy won't swag so easily. I imagine with any radical magwell opening, cold swagging will cause the frame to crack or rip apart where you want to flare it. Take a look at the frame in the above mentioned post. Thats a lot stretch for steel, especially if its not a dead soft steel.
Whats called for here is some heat. Well , a lot of it as a matter of fact. You will also probably want to have a die set to swag the shape with. One part of the die set would wrap around the outside of the frame and hold it tight, the other would slide inside the magwell area. This is at least how I would approach this. I would in all likelyhood try to put some kind of heat control in the outer die half as well to keep the heat from going to the rest of the frame, or use something like a heatstop clay. Then you heat the part, and tap the inner die piece down till it forms the magwell. This sounds easy, but is fraught with potential problems, so care needs to be exercised.
The only problem with my way, its a bit expensive if your only going to do one or a few as the dies aren't exactly cheap to make either. I dont know how the smiths used to do it, maybe they had an easier way. I'm more familiar with higher volume methods of doing things.
All the above takes some real skill, but the finishing is what makes the job, and that will take some time and skill as well, a lot of both I imagine.
As far as I'm concerned, this modification to a frame is worth a lot more in dollars than checkering. Anyone can buy a machine and do a flawless checkering job.