Well first off I agree your father in law is ignorant and jealous. He obviously is still stuck in 1960-something.....if he's really been around cars his whole life, then he obviously was very sheltered to a particular era...say 1960's/early 70's.
Better things have happened since then. If you asked him what the best Chevy heads are I'll bet he'd swear up and down it's "camel humps" (or double humps)....then ask him what the best Chevy engine was....wouldn't surprise me if he said a "327". Truth be told a set of Vortecs will beat the pants off a set of Camel Humps and a 383 will run circles around a 327 all things equal.
Now....with that said don't let his ignorance get to you. Build your car. Build it well. Then once it's going and he starts spatting off on how a "V6 can't touch a V8", call his bluff. Challenge him to a race. See if he accepts the challenge. I'm sure he'll probably come up with a flock of excuses....no biggie. Race your hubby then. In fact, make a bet on it. He did bet you didn't he? Ok good. Have him go to the track to watch you and your hubby live in action. Get a friend to video tape his priceless facial reaction when you outrun your hubby's Camaro.
That's probably the only way to make a believer out of him...that is as long as he doesn't want to believe your hubby was purposely going slow just to make it look as if you were winning. And if he still wants to continue to be ignorant, then let him. Can't change him nor should you try.
Some people in this world were born ignorant close minded tards that only want to believe what they want to. Good for them. I won't waste my time arguing with idiots like that, I just let them believe what they want to.
Oh....and here's another one.....ask him if his Chevelle even runs....

Then ask him how can you dis someone elses car when yours hasn't ran in 24 years...? Yeah.....thought so too....