I wouldn't want to do the work and expense of a salvage axle swap having an axle in good shape already under the bus. It would be well worth the extra bucks to me to re-gear.
The other reason is I don't think you'll find what you want. The lower gearing is what works best for medium duty/bus applications. You may not be able to find the ratio you want even offered new. I would get the tag information off the axle and call Inland Truck.
This is coming from someone that could do the axle swap, but would pay someone else to set up gears.
My bus is geared in the 8's on the low side, 6's on the high. Guess how I have come to these conclusions.