Stay with the auto that's already in it, it's mnuch stronger than a 700r4. Or is you do want to upgrade to an overdrive automatic, take a look at the 4L80E, which is the overdrive equivalent to the TH400 that you already have. Another option would be a gear change. Being a GMC, you should have a Spicer 70, whcih has very good gear availability if you want to change the ratio (3.55-3.73 would be a good range).
