I just built an almost identical 440 Six Pack engine for my wife's 1971 Challenger R/T last summer.
Goal was a nice driving street car for my wife to cruise and enjoy.
I went with the .30 over Speed Pro forged 2355 pistons which are supposed to be the original Six Pack type Pistons.
https://www.summitracing.com/oh/parts/slp-l2355f30/overview/?retaillocation=oh
I actually bought the entire 440 engine rebuild kit which included the pistons from Summit Racing with a 10% discount from a helpful Moparts member.
https://www.summitracing.com/oh/parts/fem-csmhp815-300/overview/?retaillocation=oh

I also used the aluminum Stealth heads from 440 Source.
After doing some research I went with a mild hydraulic Lunati cam
http://www.lunatipower.com/Product.aspx?id=1578

The engine has been run on the test stand and I hope to get it installed in the car the spring.
The car currently has a 3.23 Sure Grip rear end it, we will see how well that goes, I also have a 3.55 center section I could swap out if needed but again we are not building a race car, just a fun street driver.