mmm, not sure about silicone now. Normally silicone and petrol do not go well together.

I have just used normal pvc sheathed wire before to adapt or extend, but if doing full looms, all that fancy raychem stuff people use would be fine

I know Racetronix in the US do proper bulkhead connectors for fuel tank use etc and other loom parts...their website isnt working for me at the minute though