I replaced my dead at545 allison with a spicer 7 speed (spicer 6 + 1). The trans mounts did not matter- in my case, all mounts were on my DTA360- the trans just hangs off the back. My donor trans and flywheel were from a dt466, and I had to bore the flywheel to accept a larger pilot bearing, fabricate a mechanism for the parking brake, and make a new front driveshaft. I made/adapted a clutch linkage, and the clutch is lighter than the one on my toyota pickup by quite a bit. (a nice surprise). The 6 + 1 comes with two different sets of gear ratios- the one you describe, or the one I have, which has a 1 : 1 top gear, and a 10 : 1 first. With the 1 : 1 7th gear, I regeared the axle to 4.10 : 1, and with 10r22.5 tires, get a maximum of 10.5 mpg. I think there is a little room for improvement there, and might try 11r22.5 tires ( roughly like installing a 3.91 : 1 rear axle ratio). If you accurately calculate 15 mpg and hope for 18, you are either really lucky, or your odometer might be reading high. For a 9 1/2 foot tall vehicle that probably weighs 16 to 18 thousand pounds empty to get 15 true mpg is pretty spectacular.
Mechanically, though, the trans swap is pretty easy. The 6 + 1 has a reputation for being hard to find gears with, and for having fragile engagement dogs. The engagement dogs came either straight cut, or undercut- in the newer ones. The undercut dogs are better. The straight cut dogs are more prone to popping out of gear with a little wear. I have straight cut dogs, and am perfectly happy with the shifting, and the trans in general. Fast or careless shifting is a bad idea, though. I am also told that the recommended motor oil is essential- don't put gear oil in it. I'm not sure or the consequences, but it probably just doesn't "fling around" properly, leaving some bearing or gearset unlubricated.
If you really get over 15 mpg, I'd love to know the specific model of DT360 you have, injector pump model, injection timing, vehicle weight, etc. Going from 10 to 15 mpg would save me over $1000 per trip. An honest average over several tankfulls means more than a one time occurrence.
Good luck.- Brian
