There's so many ways to respond to the ubiquitous question, "Is it real?"
In the moment I think, fundamentally it's clear they believe it might be - and that's a compliment for you to enjoy.
And there's not a single right / wrong answer.
It's up to "you" to analyse in a split second what they wanting to hear - how deep they want to go.
Mostly the question comes over as a positive, a genuine enquiry and want to know more.
Very occasionally, it clearly comes across as a put down - in the UK some want to hear you say, "It's a kit car".
Mostly I say, " The 60's cars were Ford GT40's and there is still an active license to build "GT40's - but clearly Ford didn't sell "Ford".
Or I swap the "real" for "original" and then go on to share my personal understanding of "original" - my interpretation being an original is a "60's" GT40 built in the 1960's (Caveat - I know this isn't strictly true but it gets the conversation going)
It's then for yourself to decide how to follow through.
After a number, quite a number of, "Is it real?'" I got the hang of working out where the conversation was going and I get real pleasure out of the sharing.
And to stay on the positive as you have found yourself it is mostly guys wanting to build on their knowledge.
For me, it's been an unexpected bonus to spend time with the ones that want to talk, share and know more.
You've probably heard already,
"It's the best car here".
"You've made my day".
"Thanks so much for bringing it out and not just keeping locked away in a garage".
I've had, "It's the best car here" at the Goodwood Members Meeting - Goodwood Revival - Spa Classic and as you say the draw is off the scale given the calibre of other cars it's parked up with.
Not very well put, but I hope this gives you a bit of a frame for what to say when getting your GT40 out there.
Mine's a SGT by the way.