Few cars need a strut brace less than a 240. (Disclaimer: I believe that most people hugely over-estimate/over-state the importance of strut braces in general).
The factory braces that run back to the firewall are good for avoiding fatigue on rough roads, but most Aussie cars had them factory fitted (people in the US and some parts of Europe get excited by them because they were a relatively rare option there).
Adding a brace from tower-to-tower is not a waste of time, but don't expect handling miracles. If you must do it, something that is made of a beefy cross-section, is as straight as possible, and does not include bolts and definitely does not include heim joints will work better than anything that has any/all of those things.
The vast majority of aftermarket strut braces are purely cosmetic. If you bolt it to one tower and can move the unbolted end by hand, then you have to ask what it is really doing for the car's structure...