9 seconds on 9 inches???

brother in laws old 73 duster had a a herb mc. motor in it 451 rb with iron -1 heads a pg trans and 456 gear went 6.20 on a 9" slick with leafs... broke the ring and pinion after about 20 or so pass and ladder bar the car cage and coil overs car would go 660/670 in the 1/8 mile...he didn't know how to tune the chassis but the car was bad before all the updates..oh with redrilled axles as well:prayer::prayer::prayer: