Getting back to the original question, will it fit? Yes and no. If it is the larger 72 series bell housing on both than yes as they are the same standard SAE size. BUT, there are two different bell housings and there is one for a single and one for a double disc clutch arrangement. A double disc arrangement was fairly common on the Quadraplex as it was the heaviest transmission offered. It also had a rear crossmember to support the weight of the transmission. An easy telltale between the two is the bolt arrangement around the bell housing, there's a 1" difference in thickness in the bolt bosses and length of the bolts. This also pushes the entire transmission back 1" to allow for this clutch. This leads to another issue in mounting the transmission to the frame mounts. You'll need offset mounts on the transmission to match up the mounts on the frame or you can re-drill the frame to move everything back.
There could be another issue if the transmission in the B85 is the smaller 67 series, but that doesn't sound like what you have currently installed. My suggestion as you get into this, is dig into what you have and get all the archive info you can from the Museum that you can. Next, get all the info you can directly of both gear boxes you have. What's in the truck may not be what was installed at the factory. There's a stamped pad on the left side of the main box of the transmission, aft of the cast Mack emblem that has "Made with Tetrapoid Gears"(or something close). Clean that off and get all the numbers off it. Also, don't go just by the shift plate installed in the cab as to what is installed. The duplex would start with something like TRD-72 or TRD-720, the Quadraplex will be TRQ-72 or TRQ-720. That extra zero means it's an over gear(overdrive) transmission, 9 vs 10 speed. In the case of the Quadraplex, double overdrive, 18 vs 20 speed. There are other versions of both transmissions, like adding an X to the TRD, which makes it a TRDX deep reduction transmission or an L, which means it's a lightweight version with aluminum boxes. There's a lot of little things that without asking questions and educating yourself can cause a lot of headaches and wasted time trying to fit things together that won't go or mount up.
Is this really difficult, no, it's actually pretty simple. Once you know what you have and what you're looking at. Picking the right components and making them work together is fairly straight forward. Asking questions here can be very helpful as many of us have been messing around with these trucks for a long time. Post some pictures of what you have and ask away. It'll save you a lot of time and aggravation.
You should be able to get those pretty easily. Mack, NAPA, Fleetpride. Several Mack engines use those, END-673, ENDT-673, ENDT-675. I don't have a part number for you, but they shouldn't be to hard to track down.
Mack 573GB257A, 573GB260 maybe?
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