I was thinking about this last night & figgered I'd post it up. On the E-body cars with the rear sway bars, where the brackets mounted to the rear frame rails - they mount with 4 beefy sheetmetal screws on each side. Would it be better/stronger to use two long grade 8 bolts with a locking nut to run through the rails, or is the 4 screw method perfectly adequate?

Originally they used the four screws per side. I'm pretty sure that when you get an aftermarket bar, they come with long bolts to go all the way through the frame. The problem with that is if you aren't careful, you can crush the frame if you overtighten the bolts. I've seen where people open up the holes, install a thick tube inside the frame and tack it in place. Then you can't crush the frame.

I'd probably install nutserts in the frame and use bolts rather than screws.

its self treading screws, I would stay away from long bolts..its not needed that badly

I wouldn't think there are a great deal if stress put on those screws because of how the sway bar is designed to function.

Nutserts would do a pretty good job, I wouldn't put any type of "self-tapping" fastener there. FWIW, My aftermarket Hotchkis brackets are welded in.

If you are through-bolting it, you want to add some compression sleeves inside the frame rail to avoid crushing it. Drill ~7/8" holes, insert 7/8" compression sleeves, install bracket, put 1/2" bolts through it.

The only problem I've seen with those brackets is that he frame rail rusts out underneath and compromises the rail. There isn't enough stress there to worry about stripping the bolts and if there was the best solution would be to fully weld around the perimeter for strength and moisture sealing. Through bolts are too easy to crush the rail. While you are at it, make it so you can change the bushing more easily. Essentially make a new bracket design or modify the originals with bolts.

I've done this several times adding rear factory bars to my E bodies for the last few decades....zero issues, easy pezzy... employed none of the over thinking mentioned in other posts above, just drill/bolt done.... the one on my 71 Challenger has been on there 40+ yrs with poly bushings....zero issues

I was thinking the same thing. The bracket that fits over the unibody frame rail is beefy enough itself without even thinking about crushing the frame. I was mainly wondering if the bracket movement over the years was enough to start working the 4 screws loose. I'll leave it as is.