Mine was fabbed back in the '70s, not a kit. Downbar has a sleeved tube at the back where it joins the hoop, with a receiver cup. Quick-release pin. Probably not as stiff structurally as a welded-in downbar, bit it's a vintage car and I ain't changin' it.
There's a restraining strap on the driver's door at the hinge pillar to limit its opening, so I can reach it belted in.