Brake pads too tight
69 Mach1 power front disc, new pads, new rotors, new caliper pistons  Caliper won’t go over rotor when fully compressed  Original calipers.  - bought car new, low miles  Saw reference to similar issue on another site- many years old- but no info as to cause.  Hate to cut new rotors or sand down brakes - has anyone else fought this battle?
silly question are you sure the pistons are fully compressed? have you tried a different brand of pads?
are they new calipers or just the pistons replaced in the old caliper?
Since you stated they are new caliper pistons. I hope you kept the old ones. Then simply measure the height of the pistons, if the new ones are longer, there's your problem. If they are the same length, then your pads could be to thick?? Kinda iffy, but not impossible. How much distance are we talking about? 1/32 , 1/16 , 1/8. ??? Hope this helps if not give us some more details, and we'll help all we can. 

Thanks for the responses- the new pistons are same as old ones and I took seal and piston boot off and piston is fully collapsed— got the pads and rotors from Napa— something is surely too thick- I see where a 68 may have had .1 in thinner rotor? I really need 1/8 in  —this Mach built in May 69 so more close to 70 production .  I am a Ford dealer and ordered this car myself when I was graduating college- part of the family! Must be brake pad or rotor issue
Talk to one of the old hands, in your shop. There should be a manual, that has the specs on the thickness of the rotor as standard. Then a simple measurement should reveal the problem. Back in the day, I learned my 71 Mach1, had the same rotors, as a 71 Torino. The next year 72 the Torino became a Full Frame car, and was no longer the same, as the Unibody Mustangs. I believe they had different rotors, maybe that's what you ended up with???

Everyone just reads a computer screen anymore. They don't have a clue if the computer doesn't tell them.

UPDATE- the outer disc pad had a metal shim attached- anti rattle?- I removed that and now can get the collapsed loaded caliper over the rotor- still seams like there should be more room with caliper collapsed - but I will go with this for now.  The new rotor is .940 which I think is is ok but still don’t know what an original brake disc thickness was.  Thanks for the help- been 4 years taking this car apart and putting back together- hard to remember how it all was- should have taken more pictures!
If I recall correctly, 1969 and 1970 front brake rotors will not interchange.  Wheel bearings and spindles are different.  1970 used larger spindles, therefore different wheel bearings.  The only time I've had this happen is when the caliper pistons were not completely collapsed.

From my 1969 Mustang Ford service manuals, new brake rotor thickness is 0.935".  So, yours at 0.940" thickness are just fine.

I use Napa brake pads on my 1969 Mach 1 and they fit and work just fine.

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