agree with Greg
forget you gasser knowledge, it is useless with diesel.
factory size is just under 2" (think metric)
you can run a diesel just fine with NO exhaust but i don't like it and neither will your neighbours.
i like 2.5" with steel muffler and resonator, the older i get the more quiet i like it.
i would second the advice of rear of rear tire and preferably on the center line of the tire so the air flow takes the exhaust away from the back and out instead of getting caught in the air flow up the back making the back black as well as getting a VERY STRONG diesel smell when you have the top off.
you have a great start to a fine exhaust system, nice mandral bends.
install a boost gauge, a pyro and an aftermarket water temp gauge. i sell a plate that mounts these gauges into the center just above or below the stereo. some people like them above, i like them below. once you have that installed then record the results and post them up. we will give lots of personal advice after we know what is happening inside the engine.
the 13BT is a torquey little bugger but it isn't a V8 by any means.
bikini top on its way, didn't realize the truck was on the water. sorry.
cheers