respecting your elders

It was polite of you to tolerate the old man's story. Correcting him would have done no good, as he would have probably been offended while still thinking that he was right.
I have a hard time keeping a straight face and a shut mouth when dealing with these people. Guys with no classic car at all, calling every 340 or 440 a six pack, bragging about their POSI axle, etc.

Agreed. It is best to just listen and let them have a moment. Have heard the "back in the day I/buddy of mine had one just like this but had a HEMI..." too many times to count now. I just nod and say "cool".