it can only be 1 of 3 things.
Defective battery, needs to be load tested.. The fact that it drops to 10v tells me its no good if there are no volt drops anywhere else.
Dont believe the hype with drycell batterys. They dont have anywhere near the life of reg batterys under normal conditions, Ive seen many dry cell batteries fail and drop cells.
Another thing that alot of people over look is grounds... On my other cars with the battery in the back I like to weld to a bolt to the frame and connect the cable to that stud. NOT drill a hole and then tighten with a bolt but WELDED.
I also like to use 0-1 guage cable. For negative.
I also recommend welding a bolt to the frame and running a ground cable from the engine to that bolt..
this is the sure fire way of not having battery issues.
Now, another Item that you should adress is heat soak... I know your running long tube headers, Is your starter protected from the extra heat?