This is a good option.

Are you using brass nipples? I ask because my daughter hit "something" on her CX bike and the brass nipples on three or four of the spokes deformed and pulled through the rim preventing rim damage.

If you have trouble again, try some DT Alpine III spokes. I'm going to use them for my next wheel build because they'll fill the spoke hole in the hub better and provide more support area at that point. The 1.8 section will also provide a little bit of stretch under tension.

Brad

PS A trailer, as mentioned earlier, may very well be a good option even with the 60-70 lb. load