There is no problem using a mechanical Fuel Pressure gauge in the cab with DIESEL fuel. 20 psi tops of diesel is no threat of explosin or fire, so dont worry. Just use braided stainless steel line and you have nothing to worry about (2,000 PSI burst strength). You can use the clear plastic tubing, but i dont want to clean diesel fuel off my interior because the plastic hose rubbed agaist a screw or metal and got a hole. If a boost gauge hose gets punctured, there is no mess, but anytime liquid is involved, i just spend an extra $40. 00 and get indestructible stainess line.
When gauges fail, they stop working, they dont burst so your chances for leaks are virtually non exsistant. The isolator setups companies like Autometer sell are for Gasoline/Alcahol applications where in the rare event you did have a catastrophic gauge failure it would result in a fireball and of course, death. Diesel does not have those properties, ESPECIALLY at 15-20psi. In my expierience in drag racing and being a Locomotive mechanic, i have grown to dislike electric gauges, especailly digital ones which are annoying as hell since they are never steady. Also, senders go bad alot more often then mechanical gauges. There is a post on here now where a members oil pressure gauge went to zero and he pannicked and pulled over and it was just a bad sensor. When you have mechanical gauges, there is no question that what the gauge says is really happening. No need to wonder if its electrical interferrence, bad sender, loose wire, blown fuse, etc.
When mechanical gauges fail (Usually the diaphram) they just go to zero or stay locked where they are. Its rare that it happens anyhow. I have'nt had a single mechanical gauge failure in over 12 years. We have locomotives built in the 50's with the original gauges in them still working fine.
Digital gauges look nice and give you lots of options like alarms and such, but i just cant justify the aggrivation and cost. I can see my EGT, Boost and Trans Temp gauges on my A Pillar fine and out of the corner of my eye i can tell if a neddle is not where it should be. Just my personal prefference, to each his own.