Mike,
Bob Brashear alerted me to your post here. My wife Sue and I installed a ZIMO MX64H HO decoder in a Mack (I figure out which wire goes where and Sue does most of the hard work - disassembly, soldering, installing, reassembly, etc.)
The install is pretty simple, it's almost like the motor block was designed with DCC conversion in mind. Remove the brass rods which go connect the track pickups to the motor tabs. Cut 2 pieces of straight brass rod of the same diameter (I forget what it was) to the same overall length, solder the decoder track wires to them, and put them in place of the original wires. Solder the decoder motor wires to the motor tabs.
I forget exactly how the light was connected (I think to the motor) but I do remember they were easy to disconnect and attach to the decoder. We wired both the front and rear light wires to the single bulb. I then set a light effect (flashing, strobe, etc.) on the rear light so we can tell which direction the Mack is going(?!). This decoder fit in the bottom of the motor block.
Heed well what others have posted here about the current draw and HO decoders. We installed a ZIMO MX64 (it has a lower current rating than the MX64H) in a Hartland rail truck. It worked fine on the programming track but would not run on the main track due (we think) to a high start-up current. We replaced it with a MX64H and it runs fine. I don't know if the rail truck and the Mack have identical motors but I suspect they are similar if not identical.
Given current pricing, I'd consider low to medium end large scale decoders (Digitrax, Lenz,…) in addition to the MX64H. These would probably have to be mounted outside the motor block but there's plenty of room underneath there.
A couple of disclaimers: (1) we're mostly an LGB family (H-L-W is our #2) and most of our decoders are LGB/Massoth, ZIMO, and ESU. (2) We have successfully used ZIMO MX63/64 and ESU LokPilot HO decoders in LGB single motor block locos and a "flyer". I suspect the Buehler motors have a gentler startup current draw.
Jeff S.