Denise, do you know if it's a P-51 or a Fun Scale Mustang.40? It makes a difference. These are 2 different models made by the same company(that changed names a few times..)
The company would be either Jemco or MarksModels. DynaFlite didn't exist 40yrs ago, or carry either one of the Mustangs until probably 25yrs ago. The planes are different in that the P-51 is much more scale in shape and appearance, the FunScale still looks like a Mustang, but isn't quite as accurate in shape.
Since you say it was 40yrs ago, I'm sure you'll need the whole plan sheet... the instructions were printed on the plan sheet along with the full size plan.
Try this link(hoping it works), for a plan sheet for the Jemco FunScale Mustang.40... see if this might look familiar.
http://outerzone.co.uk/plan_details.asp?ID=5572
It's a free download, view it on your computer, and if it is the plane, just take the file to a local printer(Kinkos/FedEx/etc..), and have them output it at 100%. It will cost about $10 give or take.
If the link doesn't work, I'll come back and fix it... but give it a shot anyway. I hope this makes sense. Your Dad has to finish that Mustang!! Good luck.