This is a T29. It used the T5 105 gun and was similar to the T26E3 but lengthened to hold the massive new turret.
The the T30 would be essentially the same tank, but with a Continental air-cooled motor instead of the Ford.
the series ended with the T34 heavy tank which led to the development of the M103 heavy tank program (theres one near my house at the Tank Museum in Danville Va.- its a real MONSTER!)

with the 120 mm gun T53.
Chris