I wouldn't put it in even if was free. Too many better choices these days where you don't have to cut up your classic Mustang.

Since a 76 Mustang has under it a MUSTANG II front end, and a lot of people use the MUSTANG II front end in 65-70 Mustangs, YES, it can be done.  The big questions is SHOULD it be done?  My opinion is NO.  What people fail to realize is the 65-70 Mustang frame rails are not really designed to take the entire load of the car.  The shock towers and inner fenders and bracing were there to add support. The shock towers carried the load at the top, where forces were spread out throughout the space-frame.  The frame rail material is just thin sheet metal.  They wind up welding a sturdy MII crossmember made from 1/8" to 1/4" thick material to sheetmetal frame rails that are pretty flimsy.

But, if they don't KNOW it shouldn't be done, then what the heck, why not do it?  Ignorance is bliss sometimes.  I just watched a new Overhaulin' on TV the other day where Foose & team installed one in a Cougar, which is structurally identical to your car.

The brakes were really small on those MII cars plus were four lug.  I designed a big brake kit for the MII front ends using COBRA or 05+ GT discs, so that problem can be remedied.  Still, the handling and all is not going to be any better than a beefed up original front end with an add-on R&P installed.