I was just doing a heavy cargo flight from MPTO to LHBP. I knew I was going to need a long runway, so I asked to take off at 03R which is about 10000 feet long when requesting pushback. However, the ATC replied with 03L instead, which is about 1500 feet shorter. I literally just managed to pull the plane up right at the end of the runway. What could I do to let the ground control know in similar situations? Thanks.