I think that extensions are not that radical or high. Could be wrong, but I think they are for only increase small distances due to floor thickness's.
The 2X6's I hope are treated and how are you fastening them to the cement floor? I would use allot of liquid nails and buy a Remington 22 nail gun to shoot the nails into the wood into the flooring of cement.
The last thing you want is for a 2x6 to come loose and start bowing or squeaking. I would make sure while its opened up that they are down securely.