Those 340s mightabin 10.5 engines when blue-printed to NHRA specs back then,
But taking an original apart, and subbing in an 039FellPro, about a minute with a calculator will point to about 9.6 in the real world today.
BTW
I put a 318 cam into one of those back in the early 70s, and it was ferocious on the street in my 65 Valiant wagon. Oh, I forgot to mention that I installed 318 heads on it too, closed-chamber ones. I have no idea what the compression ratio was, I don't think I was even 20 years old at that time. I just bolted together what I was given. Man that was a combo!
I called it my Big-Bore 318, cuz it also had the 2barrel intake in it, lol. It was all 318 except the bore size and the Fenderwell headers.