I recently picked up some GT350 springs and I'm planning on putting them on my 21 GT.
Which struts should I use for this? Was looking at the pro actions from Steeda.
There are better options out there. As previously mentioned the FRP track shocks are great and specifically made for those springs. Billstein HD6 is another great option.
Go to the vendor section and make a post in Levittown Ford Forum asking Benny for a discount code for free shipping. https://www.levittownfordparts.com/sku/m-18000-f.html
I have these with Steeda Min Drop 200/800, they are just what this car needs.
You may wish to consider replacing the Rear upper shock mount while doing this work.
The OE part has a binding rubber bushing. Get a replacement with a bearing (I have Steeda).
Here are notes from a forum post from years ago...
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Rear Shock Mounts:
Brian Mac: No rubber in the mount means the shock can develop its forces more quickly and control the car better. It reduces 'the bounce'.
Putting a bearing/rod end up there also allows for bind-free articulation as the arm rotates. It's mostly a vertical motion, but there is off-axis movement enough for it to be noticeable.