Stepcraft v2 300 Z-axis too short for 3D head?

I'm trying to figure out what I'm doing wrong here. I'm brand new to all this, so please be patient with me!

I picked up a Stepcraft v2 300 in the Spring, but only now got round to building it. (My wife having bought me the 3D Print head for Xmas!) The machine itself seems to be working fine, and I've printed a few objects BUT, the Z-Axis is too short! Fully lowered, the print tip is about 3cm away from the table. I've resorted to placing a wood block under the acrylic print surface, but it's so high it can't even be clamped. I't being held down with tape for now!

Is this a deficiency in the 300 model? Can I upgrade the Z axis to the 420 size one? (It would just be the rail and spindle, assmuming Stepcraft can sell the parts only.) Will the "height adjustable" heated bed solve the problem? (Doesn't look high enough in pictures)

Please help!
🙂