Gap will depend on what type of ignition, points, durraspark, msd etc, as far as brand goes I dig the autolites!! should have read your sig, I think the MSD setup runs around .050 some where? Youd be safe there anyways, It should be in the man for the 6AL
whichever plug Roush recommends for your heads and at your CR. Gap will depend on how the car runs. I am running autolite 3911 in my Edelbrocks, which is a colder plug because of my 11:1 CR, gapped at .039
Here is some info from the MSD 6AL manual that my help you.
Spark Plugs: Choosing the correct spark plug design and heat range is important when
trying to get the best performance possible. Since there are so many engine combinations
and manufacturers, MSD does not recommend which plug or gap is exactly right for your
application.
It is recommended to follow the engine builder or manufacturer's specification for spark
plugs. With that, you can then experiment with the plug gap to obtain the best performance.
The gap of the plugs can be opened in 0.005" increments, then tested until the best
performance is obtained. MSD judges the plug gap by compression and components:
Compression Spark Plug Gap
Up to 10.5:1: 0.050" - 0.060"
10.5:1 - 13.0:1: 0.040" - 0.050"
Above 13.0:1: 0.035" - 0.045"