That car is way off, i'm thinking also it should run 10.50's. I can only go by comparison, my friend Jimmy Smith, ran 6.80's crosses to a 10.80, with a 355 flat top, @ 3,000lbs, i ran 10.80's with a 406" @ 3250lbs, both of use have 60's in the mid 1.40's and we both run .625's to .640's lift cams with 106 l/S installed at 102* i have Brodix T1's and he has the old pro top line heads.
He now runs alcohol and can run 10.50's / 6.50's, i now run alcohol with a 377" and have run 6.52 and 10.59 @ 125 mph.
I think that's the same cam that Jimmy has in his car if it's has the 106 lobe seperation.
I'm asumming you have 1 3/4" or 1 7/8" headers, also i looked up the gear ratio with your tires. If you run 124mph, @ 7,000rpm's you need a 4.37 gear, at 118 you need 4.59 gear. It almost sounds like the cam is off, did you degree it in? because it doesn't have the bottom end torque it should have.
I also have a set of old Brodix 10's RP-FP heads and they flow 314 CFM at .700 lift. i have run them yet but i have them on the 434" motor that's going in my Malibu.
I don't know exactly what's wrong but something for sure is not right, but i'll bet the problem can be found.
JMO
Zip.