The port on top the filter isn't 1/8" it's metric bolt thread and is your bleed screw. There are adapters available, but this is how I prefer to do since I like to use my bleed screw. :
I drill and tap the top of the injection pump banjo bolt to 1/8'' pipe thread, then install a male x female 1/8'' needle valve with 1/8'' tubing compression fitting in it. Boost gauge or oil pressure tubing pushed though some 1/4'' tubing to protect against cuts and kinks to the gauge. Some folks like to use braided stainless tubing, I feel it's an overkill and hard to route, but it does look nice. The stainless is just on the outside of rubber hose for abrasion and to protect against high pressure bursting, something that shouldn't concern you with just 20 psi.
For a gauge you can use a fuel, boost or water pressure gauge. I use rear connected, liquid filled, stainless water pressure gauges and make my own mount. They have no lighting but it's not a gauge you need to look at all the time. They can be mounted low for the same reason. Use no Teflon tape, it doesn't work with diesel. Instead use anti-seize or Permatex #2. Don't over tighten the tubing compression fittings, you will just distort the ferrule and it may leak. Remember you are working on the clean side of the filter, keep everything spotless.
If you can tap your manifold for a pyro tapping the banjo bolt is a piece of cake, Geno's also sells tapped bolts. Just drill from the inside to keep the hole straight and tap from the outside. You can also connect to the fittings on top of your fuel filter with the appropriate adapter to tubing if you don't want to tap or want a pre-filter reading also.
After the install start your engine with the needle valve closed then open it slowly till someone in the cab signals you that it's just reading pressure. If you tighten down the packing nut under the valve handle the setting can't move. The valve will control pulsation and buzzing and can also be shut down in the unlikely event that you have a leak. Since you are running the valve almost shut any leak will be very small.
Parts list; 1/8'' needle valve - $2. 50 from the hardware store, plastic oil pressure tubing kit that comes with the compression fittings - under $10 from an auto parts store. 1/4'' tubing for protective sleeve - less than $2. Gauge is your choice and may come with tubing. The water pressure gauges I use cost less than $10, bringing the total cost to less than $25 and a couple hours of your time.
I've installed several gauges that have been in service for over 5 years with zero problems though you may have to occasionally close the needle valve a bit more if the gauge starts pulsating.
Attention: TDR Forum Junkies 
You have any Pic's?
