I replaced the original TH350 in my 74 Nova SS 350 cid with a TH400 out of a 67 Impala. That's been several years ago but if I remember right I had to drill those holes for the crossmember to bolt to the frame. I am thinking I used the original Nova crossmember but I may have used the one from the Impala.
And 6-9/16" toward the rear sounds about right.
Yep, an electric actuator switch will need to be installed in order for the kickdown to work without having to downshift manually. I skipped that detail and shifted down manually.