Build yourself a HD 48re and itll be stronger then an allison any day. Allisons are only tough when the truck is stock. Once you mod those duramaxs the allisons dont hold up after 500hp.
The Allison is a great trans but your right anything much over 100hp from stock it starts to break the tanny down. if you are really set on the allison you can put an upgrade kit on it to withstand much more HP it just all depends on what you want to do with it.