Put it in the leanest cylinder. This is ofter the cylinder furthest from the throttle body, sometimes the closest to the fuel pressure regulator. Hopefully both.
Use a new 1/8npt tap, and work slowly using a lot of cutting lubricant, making sure to use the proper drill bit (Q drill, if i recall correctly). Since stainless is very hard and brittle, make sure you turn the tap 1/4 turn backwards after every quarter turn cut forward. This will break the chip, and keep the tap from binding. Your best bet is to use a large T-handle tap wrench and clamping the manifold in a bench vise.
Frequently check your fitting in the hole to ensure that you do not tap the hole too deeply. NPT is a tapered thread (National Pipe Taper), and if you push the tap in too far, you may not be able to get the fitting to snug up. Once you are able to engage about 3 threads by hand, your hole is sufficiently tapped.