I had the pan off on my 97 not so long ago, I see your 99 is the same.
The oil level sensor is mounted through the front side of the oil pan about half way up. To remove it, first remove the plastic cover that protects the wiring connector, then remove the connector and finally unscrew the sensor.
Of course you will want to remove the flywheel access cover as well and drain the engine oil.
The pan gasket is quite likely to be reuseable. Mine was leaking which why I had it off. As an aside, my pan is dented on the bottom so I tried straightening it out using a piece of hard wood and a hammer. The pan is extremely hard and after a few solid blows with ni effect, I gave up in fear of breaking or cracking the pan.
I used Permatex Blue Gasket Maker to seal mine, it'* a non hardening high temperature (450* F) material. I cut a small tip, about 1/16" and laid a bead over the existing empregnated sealant on both sides of the pan gasket.
A couple of pointing on not having any leaks;
Turn the pan upside and place it on a flat surface and inspect for deformation on the gasket mating surface, either being bent from over tightening or miss handling. If you notice it'* not straight, use a hard piece of wood and a hammer to straighten it.
When torquing the pan bolts, start from the bolts in the middle and work towards both ends doing the bolts on each side as you go. You should tighten the bolts in stages, at least 2 if not 3, the manual calls for 125 in. lbs of torque.
It'* best to let the gasket maker material setup over night before starting the engine.
Hope this helps...
40th Anniversary Edition
13.074 @ 106.579
FWI | 4" Intake | LQ4 | N* TB | Custom TB Adaptor | Ported SC | 2.8" Pulley | Custom Intercooler | Ported LIM | Custom Ported Heads | 105# CC Springs | Modded Retainers | Oil Restricted Push Rods | CC OE-R Lifters | XP Hot Cam | TOG | HF Cat | 2800 ST | 3.29 Gears | DHP PowrTuner | LC-1 Wideband | Engine now retired |