1-5/8 versus 1- 3/4 headers

They might work fine for what your doing then with that much motor. Sounds as if you might benefit from a little less torque and more HP.
I read a report last year or year before in Mopar Muscle (I think) where they tested several different headers. The 1-5/8 did have more low end torque than the 1-3/4. The 1-3/4 had something like 10 more HP up high if I remember right. I don't think they had as much motor as you though so yours might gain more. They also tested the TTI step headers and they were the best overall performers for a hot street engine. They seemed to be the best of both worlds.