Not sure about the 68 but in 69 and 70 the shock tower had a reinforcement plate that extends all the way around the tower rather than stopping at the edge, I've seen the same reinforcements on Cougars and convertibles even without the FE engine. Yes you can put a 390 in a Mustang that came with a small block, it fit's no problem since it was optional in those years. It will require some new engine mounts and it would be a good idea to upgrade the suspension to handle the additional weight.
The seller lists these as 69/70's but from what I have seen these are 70 only. 69's do not have the notch at the bottom & the slope on top of the brace does not exstend down as far shortening the sides.:
To answer the second half of the question, yes, it is very possible to put a 390 in a small block car. My fastback came with a 289. If you check out my website, you can see what I did to strengthen the shock towers.
Ford is famous for using whatever it took to keep the production line running. It's entirely possible that your 302 could have wound up with the big block shock towers. My 70 Mach 1 is an H code (351 2 barrel) and it came from the factory with the big block shock towers.