I know this was posted 4 years ago but I've been looking into it myself for Arkansas. We have very hot summers and the soil is well drained, but the problem we have is too much humidity and possibly too much rainfall. If the soil is well drained I don't see it being a problem of too much rain as long as there's no standing rain around it which would promote root rot. 

Another idea to think about if the pistachios you want to grow aren't working is to graft your own on a Chinese Pistachio tree rootstock since they are more suited for our climates and rainfall. You will need to do a little research though because the Chinese Pistachio only gets about 25-40 feet tall and you don't want to graft a pistachio tree onto it that grows faster or taller than the rootstock can handle. I would like to hear how you're doing as I am also in zone 7.