The "L" like superfreak said sounds like a great idea. The rulebook doesn't even call for a plate there but its a good idea. I used about a 3x5, 1/8" thick. I welded my plate all around to the rocker metal, but some folks have told me it would have been better to do some 1" stitches instead due to the thin rocker metal. I don't know if that's better but it would be nice to know!

Also, I have that same Alston E-II crossmember, and on mine, I found the driveshaft rubbed the bottom of the loop at some combinations of ride height, 4-link setting, and pinion angle setting. I can work around it, but it seems the loop is mounted higher than it should be. There's not much leeway in setting the crossmember height since it has to match the rockers, but you should mock up your axle height to make sure the driveshaft will clear thru suspension travel. I'll attach a pic of my crossmember being fitted.