The three main components to look out for are exhaust manifolds, heads, and intake manifold. There are of course a few scenarios of how this particular set up ended up in a 67 Mustang. Here are a few:
1. "X" code 2V from a 68 Mustang
2. 2V (don't know code) from a 67 Fairlane
3. "S" code from a 67 Mustang that had a 2V intake swap out
4. 2V from a full size Ford or Mercury
EXHAUST MANIFOLDS
Since it sounds like it was in a Mustang, the exhaust manifolds are probably correct from a part number perspective. Exhaust manifolds from a full size Ford would never fit. If the engine is out of the car, make sure that the correct exhaust manifolds are part of the deal.
1. If 68 "X" code, the date code will be wrong
2. If 67 Fairlane 2V, you might also have a reasonable date code.
3. If 67 S code, you might have a reasonable date code
4. If from a full size Ford, the exhaust manifolds will flat out not fit in the engine compartment. The bolt pattern is also wrong (discussed more below).
HEADS
The basic 390 head design, used in large bodied cars, had to be modified for tight quarters of the 66 Fairlane, used also in the 67 Mustang. The main difference is the exhaust manifold bolt pattern.
Since it sounds like the engine was in a Mustang (if you did not see it, make sure), the heads are either correct from an exhaust manifold perspective, or they have been modified to work. I remember back it the day helping my brother who got a 390 out of a Galaxie and put it into what was a 302 based 68 Cyclone. We needed to get proper exhaust manifolds (the Galaxie would not fit), only to find out the exhaust manifold bolt pattern was different and we needed to get the heads drilled. Certainly not Concours, but these things happened.
The 67 MPC lists four different 390 heads:
C6AZ-6049-AA --> Full size Ford all variations without T/E
C6AZ-6049-AB --> Full size Ford all variations with T/E
C6OZ-6049-AA --> Fairlane and Mustang all variations (2V, 4V) without T/E (there was no Mustang 2V in 67, but there was a Fairlane version)
C6OZ-6049-AB --> Fairlane and Mustang all variations (2V, 4V) with T/E
The 60-68 MPC lists four different 390 heads:
C6OZ-6049-AA --> Fairlane and Mustang all variations without T/E (replaced by C8OZ-6049-B in April 68)
C6OZ-6049-H --> Fairlane and Mustang all variations (2V, 4V) with T/E (before 6/1/66)
C8OZ-6049-A --> Fairlane and Mustang all variations (2V, 4V) with T/E (from 6/1/66)
C8OZ-6049-B --> 68 Fairlane and Mustang 2V/C6 and 69 Fairlane/Mustang
Based on the last two, there was a difference in the heads, so you have some work to do if you want the right ones. Whether or not you need Thermactor is part of that process.
INTAKE
This should be the easiest of the bunch, and it sounds like he has the one you need, with the only question being date code if you are looking for something correct along those lines.