Oh , I don't deny the difference in weight bias. The small block cars will certainly out handle a big block car all day long. I was just pointing out that the 390 ( all FE engines ) didn't weigh 800lbs.Yeah, I guess you're right, But....
Does that 675 lbs include the gearbox and clutch/bellhousing ? Either way, that's still 200 pounds (close) more than the small-block car, and all the weight is over the front wheels. And even though the big-block car has more weight over the front wheels, it still won't turn into a corner like a small-block car (at least, I can't get it to, and I've got way more seat-time in a GT500 than the small-block car), and the big-block car doesn't like bumps and weight changes.
I've driven both a GT350 and a GT500 at Thunderhill, and the GT350 (subjectively, at least, to me) was more controllable and much more forgiving on the surface of a smooth race track- not over cable-car tracks on city streets..... I couldn't compare lap times because we were using different track configurations, but the GT350 just reacted more predictably (or less UNpredictably)
Long story short
(Please don't make me claim that I use heavier weight oil... )
Oh, and don't get me started....because I could go into details. 1967s are a thing for me. I can even decode Shelby serial numbers....hell, the GT350 I drove was even the same color as the Bullitt Mustang, and that makes them almost the same....
When you add in the transmission and the other items there isn't a big weight difference. Some but not a lot. Yes every bit counts.