Hi Laura, My feelings are that if your planning on planting bamboo, I wouldnt worry too much as bamboo are not fussy. If you wanted to take out the stumps, I would remove the tops of these conifers, leaving about 6-8ft of the trunks and then using a monkey winch, winch out these stumps using the trunks to attach the cable and pull them over ! you can hire these winches quite cheaply and its not very hard to use them, use the strongest trunk (or a tree if one is near enough) as the anchor but as I said, its not necessary in my opinion to remove the roots if your planting bamboo !
Lannerman.