A couple things to look at:
1 - how much recoil distance are you allowing for your main gun? A shorter distance will be easier to deal with in the confines of a turret, and it will increase the weight.
2 - In TS 0.71, I think the author made an error in the calculations he's using for road wheel weight. He's multiplying the volume of the cylinder of the road wheel (since most road wheels are not solid cylinders) by 0.00168. The result, obviously, is quite light road wheels, even for the steel road wheels used during this period. I'm not quite sure, yet, what the 0.00168 should be replaced with, though.
3 - Be sure to check that the top of your vehicle (shown at the top of the Hull Design page) is wide enough for your turret.