I have had three sierra's and have had coilovers on my TOW CAR, 2.1 pinto with HT stuff, and auto and never had any trouble and used to tow stuff I was not meant to like RANGE ROVERS etc, had her over four years, towed a rangey on a two axle trailer(with brakes!!) 537 miles to scotland as an example, was not very quick but got there.
A rear strut brace is a must, absolutely essential, as towers are angled and will impart a inwards twist so need support.
None of mine ever suffered cracks or anything here, but front inner wings cracked all the time!!
Just go buy some metal, 3mm wall thickness square 1 - 1 1/2", drill two holes to fit between turret tops in boot kob done for a fiver or so.
tabetha