The purpose of installing a pyrometer is to gain information on your EGT (exhaust gas temperature).  You want to prevent excessive heat that can cause catastrophic engine failure. The valves are of most concern and if they reach 1800ºF you can expect to be very concerned. This happens when the exhaust manifold temperature rises to ~1350ºF and most people will agree to a max limit of 1250ºF. Placing your probe into the exhaust manifold will yield the most consistent and immediate reflection of the temperature of the valves. The further away from the exhaust ports you get, the more heat can transfer away and the more variant the readings can get.

This is not to say that post-turbo mountings are bad, just different.  If you choose to mount the probe post-turbo, (in the downpipe), you will need to add to your gauge reading for the heat loss experienced since the probe is located further away. This is where it gets slightly difficult. Depending on how much load the engine under, the difference can vary from as little as 50ºF to as much as 300ºF+. Since engine load can also be reflected by boost created, Jonathan (Swamp Donkey) did some testing noticed that, "At 2-5 psi there is a 50-100-degree drop. At 5-10 psi there is a 100-150 drop. At 15-20 psi it can drop up to 350, which is the most I've seen." This opinion has also been verified by testing done at Banks. This gives the post-turbo crowd a good general reference though.

It is not difficult to mount a probe pre-turbo. If you lie under your truck (drivers side, feet sticking straight out) and look up past your oil filter, you will see where the cast exhaust manifold connects to the aluminum Y-pipe. You want to drill and tap a hole about two or three inches forward of this connection (in the exhaust manifold, not the pipe leading to the turbo) and insert the fitting to mount the probe in.

Note: Your hole and tap size will be determined by the gauge and installation kit you get -- check it carefully. Some kits use a 1/4 npt fitting which would require a maximum bit size of 7/16 and a 1/4 npt tap. Some kits use a 1/8 npt fitting which requires a maximum bit size of 11/32 and a 1/8 npt tap.

I started with a pilot hole and worked up to an 11/32 hole one bit at a time. I stuck each bit in a white lube which helps the metal shavings stick to the bit as you drill, then wiped each bit off. I stopped to wipe and re-dip about twice per bit to make sure very little ended inside the pipe. Between each bit I used a magnetic screwdriver and inserted it into the hole to pick up any shavings. Although this is overkill (and a little time consuming), each time I wiped a drill bit or inserted a magnetic tip I found a little something so I figure it was worth it. Wear eye protection!

The reason I went up drill bit sizes slowly was if there were metal shavings left in the manifold, they would be small and wouldn't hurt the turbo at idle speed. After I hit 11/32 and cleaned out the hole, I started the engine (idle only) and allowed the exhaust to blow out the hole and clear any leftover shavings. I then tapped it with a 1/8 npt (which is the size of the probe fitting for my gauges), cleared the hole with a magnet tipped driver, and started the engine again to blow out any small shavings.