How high is the boom with those sails? When upright, does it come above chest high?
If not then go to a good sail maker seek advice. He may be able to adjust the sail to suit your height. You could also change brand of sails. I know a couple of tall guys who have similar problems on some sails.
As I'm sure you realise, moving the mast base back effectively raises the boom reducing back foot pressure.
Sail tuning can also have a big effect on back foot pressure. If your harness lines are too far forward you will be pulling in the sail with your back hand, this in turn transfers sideways pressure to your back foot. Too little downhaul/outhaul can have the same effect.
The easiest thing for you (and to get some correct advice) would be to post on the windsurfing school forum. The above advice I got from Roger and as usual, he was right. It might be worth you posting a picture of you sailing with the sail and one of you standing holding the rigged sail. I'm sure that would really assist with any diagnosis.
You can easilly upload pics to http://www.windsurf.me.uk/cpg133/
then attach them to your post on this forum.