RE: Big block.
I agree with Mirada, why go through all the engineering and modifications to fit the big block in, when the performance potential for the 6.1 is plenty for the street. According to published horsepower ratings in Chilton's Repair manual for the 1969 Charger, the 426 Hemi with 2x4 carbs was 425 hp -- which is the same for the 6.1. I never owned a Hemi in those days, but my recollection was that they required a lot of TLC to keep them smooth on the street. The 6.1 seems to be turn-key, ready to go 425 hp without too much tinkering...