you may want to put some 2 or 2.5" round tube in between the I Beam to help it from trying to twist. My frame is almost the same as yours and without the round tubing in between the uprights I had a fair amount of Flex. Once I installed the pipe the flex went away. I put three pieces in.
Carl.
Thanks Carl,
I've got things tied together with angle iron, dosen't show in these pics.
Also the way the ST rail is made when you adjust the belt tension and tighten the bolts down it also helps hold the Ibeams straight.
Not seen any problems yet, I'll change things to what ever has worked best in the past on other peoples.