Here is what I have found out. In 72 NHRA started the weight breaks. 6.75 pounds per cubic inch for small blocks, 7.00 pounds for big block engines & 7.25 pounds for canted valve engines. So Bill Jenkns small block Vega at 331 cubes had to weigh 2234 pounds. Big block Hemi's ran 396 cubes & had to weigh 2871 pounds. So that's 637 more pounds. Plus the Vega was narrower & shorter than the Dusters & Cuda making it a bit more aerodynamic. If the Hemi's went with the 430 inch max cube limit then they had to weigh 3117. As a result a lot of Vega's started showing up. Then Ford realized their 351 Cleveland was a smallblock & in 73 they went with Pinto's. Using the 6.75 rule. NHRA put weight on them as they dominated. The Mopar teams tried to counter with the Colts & Arrows to reduce frontal area in hopes of being more aero, but the big heavy Hemi's where nose heavy & handled poor. A lot of them crashed. I'm still trying to find out about the 340 W2 Mopars & what exactly they where up against.