It is always hard to detremine the value of a car without seeing it, but lets give it a good shot just by going on the description. Now, from what you say above, it does not have the original engine nor the original rims (rims not a big deal). Am I guessing everything else is as it should be? Meaning, is it the original color exterior and interior of what shows on the cowl tag? Is it a true Trans Am or just a Trans Am Clone? Does he have any other paperwork besides the PHS Documents., such as papers on the engine build or paint and body work? Was it originally a 4 speed car? How many miles are on the body of the car?
With all of those questions aside, lets take it that yes, the car is oriignal besides the engine and rims. If the car is as you describe it as "Driver Quality" and would not hesitate to drive it 1200 miles, and is a true Trans Am. Then I would probably put the value somewhere in the $20,000 range. Give or take $3,000 in either direction. Again, it is hard to determine vlaue without seeing the car, but this would be my best guess as to what I would pay for the car.
Here is a car that I sold about two years ago that I can compare it to:
http://www.taspecialties.com/sold/ta4173.htmNow that car I sold for $21,000 and I would say that it is comparable to what you are describing. Now, I sold that for $21,000, but that was about two years ago. Am I in the ball park?