I've fitted a fuel gauge on my escort to achieve it have replaced the plastic lines by braided lines and by the way screwed a T piece to fit the gauge (expansive but definitely the right way to do it.
Another possibility is to weld a bung on the rail to fit the transmitter or the gauge
If you wanna fit the gauge inside you need to supply an electric one (with a transmitter so no fuel inside the car) mechanical ones are cheaper but shouldn't be fitted inside.