Defiantly will be lean on at least 4 cylinders. If it where mine id either fix the carbs i was using with the 2x4 manifold and use them , or use a single 4v intake and the  single carb, then change it after break in. You didnt ask but i will add makes sure all your lifters are spinning freely in there bores. If double springs , take out the inners. Make sure its lights off and your not cracking on it over and over.

Awesome engine! That will rip in your mustang. I have put some miles on mine now and it is a handful to say the least, as it will lose traction going 80 mph bad enough to swap lanes on the highway. Have fun!

Jaime, your engine certainly turned out good! I hope you post up some track times when you get them, and stay safe on those roads! I've got a load of changing to do to my car, starting with an 8 point.

Nowhere is the intended usage mentioned, which is important. Since this is, I assume, an original '67 GT 500, I'm gonna guess that they won't be slamming gears on the strip or tearing up the pavement on roadways. If it's just going to be a cruiser, the 450's would work okay, the 600's would be better for power, and the 780's would be overkill. If you want to have some fun and not limit the engine, the 600's would be a good choice. The 780's would be the better choice if you want max power and/or want to race....but I doubt that's what you're after. I would avoid the 660's for a street car.

FYI, the repro 600's work fine on my car, street and strip. They did take a little bit of tuning to get smooth transitions, but nothing major. They are a compromise of the factory BJ/BK's, basically just standard Holleys set up to work with the factory linkage, so won't be perfect out of the box.

It seems to me that the best way to pick a carb size, is using a vacuum gauge.

If your not down to at least 1.5" at your peak shift rpm, you can use a higher flowing carb. Once you get there, or lower, then carb tuning is what will get your final max HP and TQ.

1st - Drew's 600s are easy, beautiful, work great and will feed your engine fine, do it without hesitation.  I have made over 600 HP with his 600s.  I use them regularly and they are an "easy button"  There are some later model 600 carbs that have multiple holes to allow linkage use but in general, they don't look the part and need some tuning to run like Drew's. 

As far as your Hot Rod test, that was Eric English and his buddy that showed that 450s hung in pretty well, and they did.  I believe they were actually vac secondary 465s though and although I have ran 2x 450 tunnel ram carbs on a dual quad with success, I don't recommend it.

Ironically, I built a 457 for Eric English, same guy, many years after the test.  Hyd roller, CnC ported iron CJ heads, TW, nice rowdy pump gas street/strip. We put his 465s on, made 550 HP, put a set of Drews early annular 750s on, 550 HP.   Sounds good but the real difference was torque. Drew's made over 30 ft lb more, not insignificant, but I will say we didn't do any tuning on them either because I borrowed them from Drew and I mistakenly thought I shouldn't go inside.  There was more power there, I wish we would have. 

Can we apply it to your build? I think so, your heads are better, but Eric's intake was better for peaks, so hard to tell how different it would be, especially without cam data, but I go back to the first thing I said.  Easy button.