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Agree 100% with Mert.

I could do it with a 7x - but would not.
It would involve building an external toolrest, and using a linear slide to control the toolbit.
The linear slide, new, costs more than a 7x.
The work involved would be maybe 100 hours.. by my estimate.

I could do it with my 12x, but might not.
The bore is too deep for easy work.
Ie it IS doable, but far too big/heavy/hard/slow.

I would search for someone who has a 2" thick boring bar, and this is work for industrial lathes in the 14x/3000 kg sizes, and above.
For my 12x I have a 1" one, and 1.5" ones are available.
Making one is not impractical, also.

Practical work sizes for a 7x are about 3" in steel, just like any lathe.
About half of nominal swing.
At a pinch, near nominal is doable, but very slow (everything is relative.)

Boring that much metal on a big industrial lathe will probably take 3-4 hours.
12" deep means the cuts need to be light, maybe no more than 1 mm DOC or depth, at a slow feed rate, to avoid excessive chatter.
If the total amount to be removed is say 6" or 140 mm, that means 70 passes at maybe 5 minutes per pass (could be a lot more, especially on the later ones where rpm will be about 150).
You did not mention how thick the walls need to be, I guesstimeate at 0.5 inches.
That might be 350 minutes, or almost 6 hours.
Could be more.
Of course, the first 6" depth could be taken at 2-4 mm depths of cut, if the lathe is a big enough one.

You will likely get your best price from a hobby semicommercial guy, who will do it for 10-20-30$/hr.
They will take a full day, but wont be in a hurry, and the job might tax their machines.
An industrial shop is likely to want 60$/hr x 3-4 hrs ..

Let us know how it goes, pics please ..