MS wrote:

This would be a great opportunity for someone from MustangSteve company to offer some factual data.

This doesn't happen often enough anymore.
 

The 68 should have the Midland/Ross booster (with band clamp) and the filter between booster and master cylinder.  The only suitable replacement for that is the 69 Bendix booster which uses a 9/16” spacer plate between the booster and firewall.  The booster bolts directly to the master cylinder.
On my 69 with the Bendix booster, I put a very small dent in the shock tower to allow master cylinder removal without having to loosen the booster from the firewall. It did not show and made life much easier. 
Those thinner boosters from China would not be a good choice for your car. The Bendix curved shaft booster is not too hard to come by.

Hi - been fighting the MS booster for the 68.  My car had the stock midland which was rebuilt years ago by booster Dewey and looks like the MS 68 is about an inch longer which is a challenge with the 428 in there.  Not sure of people have had any success with a smaller booster in the 68?  Been looking on search but haven’t seen anything,  Again I’ve got the factory power disc front and set back. Thanks

This is the Raybestos part.  The RockAuto page has some good photos.  In particular pay attention to the mounting hole orientation and spacing. This is for the GM left hand drive vehicles and note that the outlets are on the engine side.
=13pxhttps://www.rockauto.com/en/moreinfo.php?pk=369894&cc=1303850&pt=1836&jsn=17

I have the right-hand drive version on my car with outlets towards the fender wall.  From several searches, this part does not seem to be readily available anymore but there may be some equivalents/cross references on Ebay.

https://www.summitracing.com/parts/agb-mc390572?srsltid=AfmBOop_kUQ7gRfVoaSnzgbdJV31hiPPMmJO0n-a7OT_YhQ-OQHI4E4o

A number of the Hydroboost kits use the right hand drive version - or at least they used to.  It works perfectly with a GM Astro Van Hydroboost and I have no idea it it will even mount to a Ford booster.

The overall length is 6.75" from flange to tip.  Rock Auto had a clearance 3 years ago and I bought a spare just in case.



 

I believe it is for the Blazer, etc.  Just Google "GM 390572" and you'll get a bunch of hits taking you to the part.