Check out this post by bearcat from 2016 ...
When I looked into doing this re-wiring, I believe I figured to need a 16-pin connector.
https://fyi.boardhost.com/viewtopic.php?id=6710

Hey BobE,

I looked thru your post and noticed the pics are no longer available.  Is there any way you could send me pics?  My email is talmcgee@yahoo.com.  My phone number is 214-500-5950.

I've built several custom dashes over the years and one thing I do is try to pare down the wires to just the essentials by using junctions.  I'll use a strip of plastic with three brass lugs in it so I have a central point to wire for 12V keyed power, ground, and lighting power.  This way at the main connector I only need 3 wires for those functions.  Then all the other wires are for your senders (fuel level, coolant temp, oil pressure, tach, etc.).  I usually start by making a quick schematic using the gauge instructions.

Now on my '67 Mustang I just reworked a spare gauge feed harness I had because that was ultimately easier and I didn't have to change the factory gauge feed connector in the under dash harness, which I'd just replaced and at over $500 I really wasn't interested in cutting into it. 

I thought I did, but I just combed through my whole photo archive both in my phone and on the cloud and can't find anything.  I must have deleted those pics at some point, or they got lost when Verizon changed their cloud policy and I had to recover a bunch of stuff.  I always though some things got lost when that happened..

Basically I just laid the old harness on the new gauges with the plug where it needed to go and then started removing stuff I didn't need from the harness and splicing wires for the gauges with solder and shrink tube.  I changed my gauge layout from stock be removing the ammeter/oil pressure gauge from the cluster and substituting a tach.  I then put a voltmeter and oil pressure gauge hanging off the bottom of the dash below the radio. 

I used 2-5/8 gauges and 5" gauges.  To mount the 2-5/8" gauges I simply removed the plastic to the limit of the chrome bezel and they fit.  I had to get a bit more creative with the larger gauges.  They say they are 5", but in reality they measure more like 4-5/8".  I used a die grinder to remove plastic until they fit.  I built an aluminum plate from 1/8" stock to mount everything in the rear.  I'll send you a pic of the setup in the car.