Looking for new sway bar. Can I go too big??

Unless all of you are planning on having a custom bent sway bar, old Mopars with torsion bars are really limited to (4) manufacturer's and only (2) of those fit without cobbling **** to get them on. There is really only slight handling differences between the (3) manufacturer's sway bars and the handling characteristics they produce for the average street driven car. Last I checked a custom bent bar runs around $500 +.

And being a Dad, you are probably more concerned about improving the handling some on your Father/ Son project car so he doesn't get out braked by a modern sedan in front of him that's slamming on the brakes, or taking a corner in an old car with body roll issues a new car takes with ease
Generally if you have stiff torsion bars/ springs you can use a less stiff sway bar. If you run softer torsion bars you can run a little stiffer sway bar. And don't think this means you can keep your Slant 6 torsion bars and run the biggest sway you can find. It needs to be a little closer set up than that.

There are only a few people on here that can tell you why they like their setup when pushing the limits, and that's because they autocross their car. So where one person likes Hotchkis another person will like Hellwig and then there are people that like stock Mopar sway bars... Aftermarket larger diameter sway bars are usually always an improvement over stock sway bars and will work for a majority of purchasers. But to complete it, you really need to upgrade your shocks too, and I favor Hotchkis in that area.

Have fun with it. Teach him some skills. We are building the next generation that loves the same cars.
Your second and last paragraph are what I'm after, appreciate your input also.