Yea the chasis will flex when you tighten the bolt.. thats what happend to our robot this year. To fix it all we did was just cut aluminum blocks, drill a hole for the bolt to fit in, slide it into the channel of the chasis, and then tighten.
Neat Idea.
Andy Mark Sells something that tightens chains though.
http://www.andymark.biz/am-0286.html