I brought mine into the cab through the floor and mounted the PCM and fuse/relay box under the passenger seat. You can see it in my 70 K10 build thread.
I drilled a hole for either 1-1/2" or 1-3/4" grommet (can't remember now)
PCM and fuse/relay box goes under passenger seat. I was going to keep the OBD2 port there too, but I decided to move it under the dash.
I used Blazer tilt brackets so I would have access to the PCM if needed.
