The mounting brackets for this bumper completely dominate the available space on the end of the frame, which precludes running the OEM shackles or pintle mount. You wouldn't be able to sandwich the bumper mounts between the OE mounting plates and the frame because they are gusseted. If the end goal is to preserve as much of the original equipment as possible while adding swing mounts, you would be better served whipping something up from scratch with cardboard aided design. This one is 3/16" plate. 1/4" could really take a beating at the expense of even more weight. Depends upon what you're doing with it.
Now that said, you can reach out to customer service at Chassis Unlimited to see if they'd be willing to work with you on something.
britny@chassisunlimited.com was my contact. They seem to do bulk production runs, so I don't know if they have the inclination to do custom orders, but it doesn't hurt to ask.
I had planned on installing a relay triggered by the reverse switch to run the flood light, but setting it up to plug into the extant trailer harness definitely sounds more expedient. Also, I've been removing odd wires, not adding them, for a year now. Best not to reverse that habit. Thank you for sharing your knowledge on the neutral switch. Spent a lot of my career working on newer vehicles and never knew a NPS to be adjustable.
One more thing: This bumper stands four feet behind the rear axle and weighs about 300 pounds loaded, plus the load generates leverage due to being an upright at the extremity of the frame. I'm not a mathmagician, so somebody with a chalkboard will have figure those physics. What I've noticed this week is that the rear end wants to keep going once it's picked up side load. In other words, keep both hands on the wheel while turning at speed. I believe this handling characteristic can be corrected with a rear sway bar, obviously with quick disconnects. Looking into it, the aftermarket hasn't impressed me. Hoping to find an OEM GM unit. Something probably exists given most of the stuff under these trucks interchanges from 1963 to 1991.