Typically the stock arm is cut in half , Box tubing that will fit inside the stock tubing is your best method because the weld will be a " socket " weld , the weld is there to hold everything in place , the square tube extensions inside the factory tubing is what gives it the strength . You need tubing that fits snug , and if you are adding 4 inches in length , use a extension at least 12 inches long so you have plenty of steel inside the factory tubing to make it strong . The stock shock mount location is about 12" from the centerline of the swing arm bolt , my plus 4 shock mount sat back further than I liked at 14" I'd say keep it at 12" or move it forward an inch . This will cause the rear of the blaster to sit a tad higher , but will stiffen up the feel of the stock spongy shock . Your friend could get fancy and make a adjustable shock mount so you can adjust it to your riding style and subsequent weight gain from pounding junk food and mountain dew all day after riding the f**ck out of your blaster .
