Hp7500A printing problem with A3 pages

[mkmkmk]

I've just bought the Hp7500A, I'm trying to print Autocad drawings, how or can I change the margins for top and bottom of an A3 page, at the moment I lose 20mm top & bottom, not much chop for drawing with a tile block. is there another driver that can be used or someway to over ride this problem?
Can drivers be hacked?