Seems to me the guys with a particular X275 '68 Dart ran them and had a record of 4.17 in the 1/8th at the time. So I know it can be done. The latest thing is to put Caltrac Sliders on them. I am considering this for mine. And yes, mine are relatively flat as well. But I bet my 1.33 sixty foot is nowhere nears yours. I am not happy with the ride they give, or the inconsistent 60fts.....I actually like the RMS rear bolt on 4-link system even more...Wondering if that could be adapted to my tubbed POS...I would call Calvert and get one of their techs on the line....

'70 Cuda,...605 EFI Hemi Street Car (6.20 best pass, 1.33 60ft)
https://www.youtube.com/watch?v=WYw6RA-k5Bk (6.25 at 108.75mph from inside car)
https://www.youtube.com/watch?v=3zQEb9uxFng (6.25 at 108mph from outside car)
https://www.youtube.com/watch?v=JCvfzsC4NgM (9.9)

'66 Barracuda AWB Stretched nose Blown 440 Car in build stage

'71 Duster Drag Car 400 Low Deck 512 best 6.002 at 115.44mph
https://www.youtube.com/watch?v=Znuo3jMUXTk