Play by ear on the pin. Just take a look at the slope it has right now and try to keep that same profile, but further into the pin. Now that I know theres really not any tight tolerances to be held, it should be a breeze. Just make sure its smooth all the way through, and take it down to a shiny finish. I'd venture to guess it wouldnt be so good if that tiny pin would bind up on the cone.
The cm sizes, or turbo sizes, have nothing to do with the output flange on the exhaust housing. Your current turbo is somewhere between an 18cm exhaust housing and a 21cm exhaust housing, I think. Anyway, the 16, 14, and 12 all have the same size flange to accept the downpipe. That flange is going to be 3" on yours. Mine is a 3. 5" and the turbo is an offbrand equivalent of an HX40... kinda. The true HX40 does have a 4" flange though, as I recall. And yes, I believe the standard flange on the HX35 is 3. 5". Don't quote me on any of this.
Anyway, there are a bunch of measurements that go into how your turbo performs.
First are the pin wheels and housings. What putting a smaller exhaust housing on your turbo means, is theres less room for your exhaust gas to get around the pinwheel, therefore it builds pressure. The same amount of gas is forced through your turbo, but through a small area. Physics, kiddies. This means the gas is moving at a faster rate. The venturi effect. Now since the exhaust is moving at a faster rate, that means your pinwheel is spinning faster. Which in turn, means your compressor side pinwheel is spinning faster, and providing more air. You can get this same effect by putting a bigger pinwheel in, in theory. But considering the bore that the pinwheel sits in is machined roughly to size, getting a bigger pinwheel just translates to a whole new turbo. Think HX35 vs HX40. The HX35 is a smaller all around turbo than the 40, but the main difference, at least on the exhaust side, is the pinwheel. My new turbo, the outer dimensions are not much bigger than the stock WH1 that the truck came with. You'd look at my turbo and think its not much of an upgrade. The difference is, my turbo has a massive pinwheel compared to the stock turbo. My exhaust housing looks to be roughly the same size as stock.
Heres the catch. Putting a smaller housing on makes your stock pinwheel spin faster, true. Putting a bigger pinwheel in stock size housing makes the pinwheel spin faster, true. The difference? When you put a smaller housing on, you in essence decrease how much gas can squeeze through with the given pressure from the motor pushing it. A bigger pinwheel with a stock size housing allows you to force roughly the same amount of gas through AND spins the pinwheel faster.
Why can't you tow with a 14cm? The smaller the housing, the more pressure build up you get. Which, as I said, spins the pinwheel faster. But the other problem is you can actually build too much pressure too fast. When you motor is under a load, it puts out more smoke. More smoke is being forced through the turbine, and pressure builds up. Now put a trailer behind it. Big load on your motor. More smoke. More pressure. 14cm housing? less room for that pressure to get out. Eventually your gonna blow up your pinwheel, or some terrible other twist of fate that will leave you stranded and upset.
As far as the outlet of the turbo, that flange that the downpipe fits up to, bigger is better. For the most part anyway. A bigger opening lets the gases exit out faster and lets the pinwheel spin faster. Why is this different than the housing? Picture a garden hose. Squeeze it together at one spot. The same amount of water now needs to run through a smaller spot. In that particular spot, the speed of the water is faster. Nevermind anywhere else on the hose, because yes, it will slow a bit. But where you squeezed it the water is running faster in that kink. Now put another kink in it further down the line. The water will slow down everyone else on that line. Yeah, it'll speed up at the new kink, just like the first one did. Picture the first kink is your exhaust housing. The turbine is in it. Without the kink, the turbine spins normal speed. Now put the kink in. The turbine speeds up. Thats all you need. But if you put another kink in further down, it'll slow that turbine down again. The same theory works if the second kink is upstream from your turbine... the exhaust manifold. You want gases to be free flowing eveywhere except for at the turbine, there you want to restrict them... somewhat.
In our next lesson, we'll talk about the compressor side. Right now though, my hands are cramping and my eyes hurt.
Attention: TDR Forum Junkies 
its my everyday driver and i will most likly do some good towing with it in the future, but for now, i gets me to school, work, and an occasions 6+ hour road trip. what housing is the most practical??? for what ive described. 
Used to be 150 for a 16 housing. I'd check around some. BD, Diesel Manor, etc.
