Hi Jesse, I will give you my 2 cents worth on this topic and hopefully it will help. Yes I have built several of these California haulers before... I have also built several of the revell Peterbilt 359 snap tites and they are easy to build with good detail. What I like to use for the stretch is use pieces of original frame rails from the same kit. easy for me since I have plenty of those. I usually try to make the cut where there are no cross members involved and usually under the sleeper if you plan on using one. I have used some round sprue stock for the driveline or you could use some round plastic tubing from local hobby shop in that size. What i would do is cut the u-joint ends off original driveline and put them on new driveline. Also as for covering the new lengthened area I usually use some diamond plate from Don Mills or just use some plastic sheeting and cut to to size. Length and width. If I use diamond plate I will baremetal foil it and If I use regular flat plastic stock I sometimes paint it same color as as the frame or what ever your choosing might be...Hope this helps...I am sure others have their own way of doing this, ... Some when they cut the frame rails cut them at an angle on both ends,,,,I usually just cut them straight and together at the same time...Make sure you re-inforce the inside and over lap the seems for added strenght...Good Luck...Here is a picture of my " Project 350 "