IMO
7 or 10 will be fine street strip
I have the steel retainer mid level spring , dual spring,on TF270s .
I use Dougs D452 headers , great fit .
In a 68 Plymouth B body

Tex

New best ET 10.259@129.65 .
New best MPH 130.32
Finally fitted a solid cam,
stepped it up a bit more
3690lbs through the mufflers
New World block 3780lbs 10.278@130.80 . Wowser 10.253@130.24 footbraking from 1500rpm
Power by Tex's Automotive