mark
+1y
^ yea, you can do it many ways.
but what you can do is buy the 4link, and base the mounts off of the 4link you bought. thing with SD, they will ask you the lengths you want, they dont really have preset bars, but they can make the bars as long as you want them to. so you may want to take advantage of that.
take your bed off and lay it out in your mind what you want to do, then go from there and do some measurements
the gastank thing, many of us here pretty much just used 1x1 boxed tubing and ran them across the framerails and welded them. the tank then bolts into the 1x1 tubing. then like slammed83 said, you will have to extend the lines
goodluck and don't be scared to ask questions.