The speedometer should be a simple splice thing. Just get the VSS output from the new 6.0 PCM and splice it into the lead going to the speedometer. Then, you'll have to just adjust the tire size and gear ratio in the tune to get it dialed in.

Next, the tach module is a different beast. To get it to register on your tach, you have to do the following. Take the tach output from the 6.0 PCM and splice it in to the tach gauge input (should be a white wire). The way the gauges is set up on the new PCM is there is a ~5v source "somewhere" in the harness that powers the gauge. To make this work on your OBS vehicle, you have to splice in a 12V power source and a resistor (I'm not for sure off the top of my head of the Ohms) to get the needle on your gauge to start to bounce. That was the thing that threw me off for a while was the 12V source spliced into the Tach input wire. Lastly, to get it to read the correct RPM's, you have two options. You can either use the tune to set the Tach output to 3 instead of the default 6 or vice versa (again, I can't remember off the top of my head) or if you don't want to go with the tune route, you can purchase a Baker Electronix signal converter (and I have one for sale for cheap) and splice it into the Tach input and adjust the settings on the converter to make it match your actual RPMs. Either way, it may sound like a lot of stuff, but its really not that bad.

I'll try and dig up the resistor information as well for the Tach.