Mack transmission and rear axle combinations

I'm looking at a Mack tri-axle dump truck.

I think the ratio on the rears are 5.73.

These trucks used the T2060 transmission, because it has a .60 Overdrive, and even at that they really only did maybe a little over 60 miles per hour, from what I understand.

Originally the truck had a 300 and it was set up this way. It now has a 350 in it and an 8ll.

They did not realize about the low gearing when they put in the 8ll, and the truck can't get up even to highway speed, so either the transmission has to be changed or the rears have to be changed.

If I buy the truck I would prefer to change the rears, unless there is an auxiliary transmission or some other Mack compound transmission or something that has a very high overdrive ratio like .5.

Is there an auxiliary transmission that would work with this?

If I used one with the T series Transmissions, that would give me a better overdrive and I'd be able to split the gears I have, and keep the rears that are in it.

If anyone has any other idea about the gearing please let me know.

Can the camelback suspension be taken out from under the truck, or does it have to slide out between the frame rails?