Shaun,
This question should provide some lively discussion! First I would expect all 428 “R” code Mustangs to have the welded shock towers as Scott indicates. Most noticeable by the welding on the forward and backside of the shock tower, close to the engine. Easy to see in a small block car, but not so easy for a big block. They have “some kinda full” engine compartments.
Which brings me to: In my experience I have seen “J” code cars with welded shock towers. They were true GT cars. It seems that the welded shock towers were part of the heavy-duty suspension that comes with a true GT. I have also seen Marti identified “S” and “A” code 67 Mustangs with the “competition suspension” option that did not have the GT option. They also had welded shock towers. The competition suspension option is closely related to the GT suspension portion of the GT option.
So that said, again from what I have seen and learned, it seems that the reinforced shock tower towers followed the heavy-duty suspension more than the engine size. Of course many or most big blocks were GT’s and very loaded with options. And especially 428 cars!!!
I bet there are a lot of true GT owners on this board that will shed even more light on this. Curious if and of the X code cars on the site do not have the GT option. If they are reinforced it lends back that all big blocks got welded shock towers whether they were GT’s or not.
Another anomaly of this Mustang hobby that can defy even the most thorough reseach!!
Rob