I have the Leinbach 7200 as well. However, after digging about a hundred holes I gave up and got a Worksaver Post Driver to fence in 3 acres. With 40 acres, you are going to be drilling a whole lotta holes. /forums/images/graemlins/shocked.gif
The problems I had with it were as follows. Some of these are related to the Leinbach unit (Items 3 - 4), some are related to post hole diggers in general (Items 1 - 2).
1. Get a bunch (several boxes) of grade 2 bolts to use as sheer bolts.
2. Carry tools (pin and hammer) to drive the remains of the bolt out when it sheers.
3. With 40 acres, I also recommend getting a second auger and a set or two of the cutting edges.
4. Get a box of the bolts that hold the auger on the shaft. Since the drive shaft is round and it's only held on by 2 bolts, digging puts a lot of stress on the bolts. They tend to deform, then they're real hard to get out.
My guess is that you're going to be very frustrated using a 3ph post hole digger to install a fence around 40 acres. Digging the holes, setting the post, then tamping it in will be quite a bit of very tiring work. Mark a couple of holes, dig the holes then set the posts. Don't do what I did, dig all the holes then start setting posts. /forums/images/graemlins/crazy.gif I found out the hard way that the holes tend to fill in over time. By then I had the driver, so it didn't matter. /forums/images/graemlins/grin.gif
I don't want to discourage you, but if I had to do it again, I would have never bought the post hole digger to install a fence. It's probably OK for a small number of holes, but I had to install 187 4" x 4" x 8' PT posts and 40 T posts. Hopefully, the soil in your area isn't like it is in my area. I got advice to use a driver, thought I could do it with a post hole digger. The post hole digger was just too frustrating and it was taking too long.