Your steering gearbox is bolted to the front axle, common with most intercity buses like your MC-7.
Because of it's vintage, most likely you've got hydraulic-assisted steering, something different than what you're used to in automobiles. Crank the front wheels over and look at the axle & steering linkage. If you've got a large hydraulic ram attached to the drag link, it's hydraulic assist.
Later model coaches use a Sheppard steering box, which is a true "power steering", similar to cars, and yes, you can retrofit your MCI with one.
Best deals are from Nick at NIMCO in NJ.
Oh, and there are no steering gearboxes that mount "inside the frame" for your coach, because your bus does not have a frame - it's a monocoque chassis.
FWIW & HTH. . .
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