Well, uh, if having the original engine wasn't of significance, then I guess this thread wouldn't exist.
Unfortunately many 'Shelbys' have cloned or facsimile-tribute engines, not the end of the world, but important in terms of value and originality, and your concerns on your friend's behalf are well placed.
Best to keep it fact-specific.
With the limitations you have, you could check the balancer and front cover. These dated items would typically not be serviced, as perhaps a carb or distributor could be over the course of 55 years (although balancers and cylinder heads were created in batches sometimes months away from an engine's assembly).
Your best bet is an analysis of the block stamping. As mentioned, block stampings are very nuanced from the fender stampings.
Any further feel free to PM.