I agree w/TKO on the assembly manuals.
First what year Mustang is this about?

Following the question list, here are my suggestions.
 
1.  I would install the clutch/brake pedal assembly first. (Maybe after the insulation)
2. Yes, install insulation first.  Are you using factory replacement insulation or aftermarket?  Factory replacement would likely have all the cutouts installed already. 
Is insultation under the AC unit to firewall?  I would think so, the assembly manual will help with that.
3.  A hydro boost needs to be installed and plumbed.  Should the steering column go in first?
Yes, I would also suggest the engine be installed prior to fitting the hydroboost, this will allow to see any other potential interferences.
4. When do I start installing wiring, does wiring go over insulation, or under, to protect it?
I would place the wiring over the insulation.
5.  Do we put dash pad in before or after wiring?  How about AC tubing?  Does it go before dash pad wiring?
I would do the dash pad after the wiring, etc.  Should make for more visibility and installation.
6. I started to install air and have OEM vents, side and center, but not sure how to mount into dash frame. 
Sorry, I don’t have any experience with this item. 
 
Hope this is of some help.

I've never had the assembly manuals but on this last build I used them and they were great to have.  My buddy gave me the manuals.  I'll be buying a set for the 70.

I would like to extend a BIG thank you to everyone responding to my inquiries.  
I did acquire assembly manuals as suggested.  More importantly, I appreciate the suggestions as to how to sequence doing things.  Assembly manuals are helpful but suggestions from experience is even more important.  
Thank you again!

I do have another specific question:
What are you guys using for insulation (floor and door)? 
Dynamat seems to be a good product, but expensive!  Is there anything that can have similar results?  
The car is going to be used a few times in the summer and going to some events.  It would seem putting money into functional components vs insulation, would be more logical.  
Is there anything almost as good but less expensive?  

Thank you!

BobE wrote:

Over twenty years ago, I used “Quiet Ride Solutions” sound deadening and insulation kit made for my fastback for the passenger compartment.  This excluded behind the rear and trunk.
 
A few years ago, I used “Noico 80 mil" (2 mm) Sound Deadening Mat & "Noico RED 315mil" Sound Insulation, purchased through Amazon, for behind the rear seat and trunk.  These came in squares that one can cut and piece together to fit.
 
I can’t say one is better than the other, and I have no compassion with Dynamat.   
My only suggestion would be to buy the custom fit for the firewall and maybe the floor as there are many curves, recesses, etc. to deal with.

My last 10 years doing the aeronautics  job was getting the cockpit electronics in the f35 to survive 135 dbn noise.  I found lots of data about "vibration dampening ,, ie Dynamat" and sound blocking.   my short take for my mustang was to use Dynamat ((lowers panals vibration generated noises)  on 10 %  of open spaces of panel.  On out side noise,, ie tires, wind , turcks passing.  I found sound Blocking material worked best,,,  for low freqs I used 1/2 inch thick mass loaded vinyl.  High feq I put 1/4 in thick foam on top of vinal.  I made paper templates of sections I covered.  The sterio/ music industry has lots of data on how to block sound.  There is lots of noise test data showing outside genererted noise goes thru dynamat ???   THEY DONT LIKE THIS.  ??  I didn't cover everything need but my starting point measurments in the interior was 84 dbn at 70-75 mph.  After install is 73-78 dbn 70-75 mph.   Outside wind speed/direction has a lot to do with generated noise.   My guess was to cover/cacoon the interior shell.  after a few years it has appeared a lot of noise energy is generated by tires that I thought were blocked.  I research Mercedes Enterior noise and they block wheel wheels with simialar system I used. I didn't believe it (go figure)  so I went around and "felt up" wheel wells on new modals  ( i know I'm a sic engineer)  AND yep found some kind of foam covering everything.   So I have  lots of noise still around all windows,, just some more fun to work on the emberglo girl.