I say put one in. If you don't like you can always cut it out.
I have a 10 point cage in my chevelle (no knee knocker bar under the dash).
It is 1.75" DOM .120 wall and the halo bar is tucked high and nice. The cage is through floor pan and tied to the frame. The Kirkey seats are a pain to get in and out of but they sit nice and low. I feel secure when strapped in with 5 pt belts. Here are a few pictures..it is a little slim on creature comforts and style points
I have kids but only my son (8 yrs old) has rode with me as my daughter is younger and not big enough. I have to use a bottom booster seat for my boy and shorten up the harness for him. If I had a back seat and no cage I am not sure I would feel comfortable with my kids back there anyway.
After all that is it a pain? Yes. I drove it Today and it is a pain to get in and out off. The 5 pt harness is a bigger pain imo. You can't have them pulled down all the way and make safe lane change on the freeway. I have thought about making the down bar with a curve, change seats and add 3 point seat belts as option for street driving. I only have one hobby car at the present time so having a track only car is not worth it for me. I want to get the max out of my $$ and time. Like said earlier there will be pros and cons with a cage in a street car. You are building a nice car so I would do a min of a 6 point