Ok. I’m looking to do my suspension and I see that there are ones that go to 1 1/4. Should I just go with the stock 15/16 or go with the larger diameter?
If you have the stock sized bar, I might first try a new stabilizer link kit and stabilizer attachment bushings using the polyurethane bushings. These are much stiffer than the factory type rubber bushings and will deflect less, thus in effect stiffening the suspension.
You may not gain much by going with a larger front bar by itself. You want to balance out the front with a rear stabilizer. If you go too large and stiffen up the front suspension, it can affect steering - under/over steering and now you have a dangerous condition if you are not ready for it.
pointed out, you may not notice any big difference without balancing out the suspension as viewed as a complete upgrade - stiffer springs and adding a matching rear sway bar.
If me and you have no plans on swapping springs, I would first try the polyurethane replacement bushings to tighten things up. PST is one manufacturer that offers these as polygraphite which I feel is a little better than the polyurethane type due to the graphite. I used their body mount bushings. https://p-s-t.com/i-23159970-polygra...ngs-15-16.html
I have also used the less expensive Prothane polyurethane bushings which are currently used on my '68 Lemans build having the larger front sway bar. PST at the time did not offer the polygraphite parts for the front sway bar. https://www.summitracing.com/parts/ptp-19-1133-bl
If that works, next add a rear sway bar. You could use the factory 7/8" rear size bar. Keep in mind that the later GTO's got heavier, and the bar diameter is matched to the weight as well. The '65 body is a bit lighter so going with the later heavier bars or the big one offered by aftermarket companies may not be what you want as it could be too stiff.
Below are two options for sway bars. You would have to drill your own holes in your lower control arms, but not too difficult. I would get the "shims" for the sway bar and this I assume would give you the correct bolt hole spacing for either.
It appears that you can get the bolts/nuts/inserts for use with your factory control arms. Not sure if it is part of the purchase or you need a separate kit.
Here is a 1" diameter which will also work. You may need bolts and nuts and if it did not have them, make some tubular spacers that go inside your factory control arm so you do not overtighten and crush them. You can read the reviews. https://www.summitracing.com/parts/o...tiac/model/gto
If you do not feel comfortable, or don't want to drill your lower control arms, you can purchase them new with the holes drilled and "boxed," and new control arm bushings. https://www.opgi.com/gto/KR0445P/
You may or may not need a shim kit with these if your sway bar fits tight within the lower control arms, so I would wait until I installed them before assuming I needed then and did not - just saves a few bucks.
Just a few things to consider and look into.