Joe, Welcome to the best place in the universe for classic Mustang and Ford knowledge.  We're glad you found us.  I'm no expert, but I'll break the ice.  For today, and all your tomorrows, you should find out what previous owners have actually done.  The front spindles should have some casting/part numbers cast into them, usually on the engine side of the vertical part of the spindle between the upper and lower control arms. If you found those numbers and reported back here, we could probably tell you what previous owners have actually put on that car.  As to the rear end, Ford first produced the 8.8 rear end around 1983, so it is unlikely (but possible, if narrowed) that you have an 8.8.  The Galaxie was a full sized Ford, so a Gal rear end would also require narrowing to fit in a Mustang.  Granada is quite possible, as the width is very similar to Mustangs.  Most likely, it may very well be the original Mustang rear end, especially if the car came with a V8 engine from the factory. 

Now to help address your immediate problem.  I don't believe spacers will help you, here (bandaid, and not a proper solution).  If there are not enough threads to properly bolt up a wheel, there are not enough threads to bolt up a spacer.  Spacers may well complicate your wheel fitment, depending on tire size, etc.  Perhaps the quickest and best solution is to have whoever will be doing the work remove 1 wheel stud from the front and back of the car, take them to the parts store to match the size and buy some longer studs, press them in and bolt the wheels back on.

BTW, be sure to check the date codes on those tires.  If you are planning on taking some trips, replace your tires based on age, NOT tread depth.  And if you like road trips, we're having Mustang Steve's annual birthday BASH in Michigan next month.  Drive it on over. You won't regret it!

Once again, WELCOME! and let us help you keep that '66 safely on the road!

GO CUBS! (couldn't resist.....)

Last edited by TimC (5/15/2019 8:54 AM)

Thanks for this Tim/MS.  Yes knowing what is in there would be helpful.  Notes in the files I have suggest 72 Granada (which did not exist in North America) and a 68 Galaxy.  Supposedly it has the Windsor from a 68 Galaxy in it.  The rear end was replaced a few years ago and the guy who did it said it was a rare 8.5 -- which did exist but is not well known.  They found new old stock parts and it is working great.  
I have ordered 3 inch studs with a .625 knurl diameter and .4 or so knurl length (Summit Racing).  These are similar re the knurl size to those that fit most Fords built in this period (knurl size for the Granada conversion studs at Mustang Steve are .623).  I put on the styled steel wheels after an accident at the suggestion of the guy restoring the car -- he sold them to me cheap as he had them sitting in storage.  However, he never mentioned the precarious installation and the danger he put me in.  I am pissed.  
Re clearing the caliper, the wheels are in the car now and have been for four years.  The front wheels have the spacers in them and the problem is the thread on the studs -- they seem to clear the calipers now.  Not sure how all of this has worked but it looks as if all I need is longer studs.  
BTW, I am told that to replace the rear studs the axle must come off -- is that right?  Sounds like a lot of labor.  
Thanks for all your help.  The car is not original at all -- it started life as a straight 6, etc.  All of it, from brakes and steering to engine, etc. are taken from Granadas, Comets, Galaxies and all that.  It has been a great education.
Joe

Bolted to Floor wrote:

Welcome to the site Joe.
Typically, the factory length studs can be replaced without removing the axle shafts. Remove the brake drum, find the spot with the most available room and drive them back with a hammer. Insert ne and use your sacrificial lug nut to pull them in. A rod coupling would be even better.

A 3" stud could very well require the axle shafts to be pulled. Removing them is not that big of a deal.

 
Actually, you should NEVER HAMMER A WHEEL STUD OUT WHILE THE AXLE IS IN THE HOUSING. This is a sure way to put flat spots on all the bearing balls!!!

An easier method is to purchase a Harbor Freight ball joint removal tool, about $20.
It works with leverage. You may need to widen the slot so it fits around the head of the stud, flat against the back of the axle flange, but it will pop the studs with ease.   I am not a proponent of Harbor Freight TSO (tool shaped objects) the majority of which are total junk, but that ball joint removal tool is a good one. I used mine to replace all 20 wheel studs on my 2007 Mustang and have popped many a ball joint and tie rod loose with it.