Installed them on a Dart and Duster using 2 x 3 square tubing. Opened up the end to encase the rear spring box. They did not go into the floor but lay flush against the floor pan, then tied roll bar into frame connectors.

Tony

70 AARCuda Vitamin C
71 Dart Swinger 360 10.318 @ 128.22(10-04-14 Bakersfield)
71 Demon 360 10.666 @122.41 (01-29-17 @ Las Vegas)
71 Duster 408 (10.29 @ 127.86 3/16/19 Las Vegas)