you are on your 3rd... you are either hard on the trans or the builder sucks.. sorry for being blunt..
you didn't list what type miles you got out of each rebuild.. ??
I'd maybe step up to a 4l80e.. basicly a th400 with a o/d
or install a th400 if your milage per gallon isn't that big of a deal..
might try not use'n o/d when tow/pull/loaded up.. o/d is the weak point in those..
I'm asumming (dangerous I know) that you have a cooler.. and the trans death is from other things than lack of servicing..
good luck... many trucks with tons of miles on those 7004r's..
but upkeep and t.v. cable adjustment have to be correct and followed..
maybe go to the 4l60e and get the v.t. cable gone..