I have tried the two singles at the bottom before and it gets pretty ugly, also the top is not actually balanced as there is more load on the right hand side, and by putting the blocks on the outside you are giving it more leverage to twist.

Basically you will always have uneven loads in this system whilst you are running the blocks on the sides of the boom. the best way we have designed (yet to build) is to have it all in line. 3:1 inline (block with becket at bottom ans singles inside boom) at the back and 3:1 inside the boom, this should be very free running and gives enough purchase.