Seems to me those should be more than adequate.  i have a re-purposed 
geologist's rock sample desk which I use for hardware and tool storage. 
The trays are 2in deep & simple box construction planted on 1/4" fir 
ply, which overhangs the boxes by about 7/16". They slide on 1/2" x 1/2' 
aluminum angles spaced 3in apart to allow for flexible storage. With 
paraffin applied to the aluminum & plywood sliding surfaces, they work 
famously even when full of heavy stuff. They were good enough for rocks! 
Yours should do fine.

FWIW
Don


On 4/18/2014 1:27 AM, Malcolm Thomas wrote:
> I did plan to use 19mm x 19mm jarrah for the drawer slides, however i realise
that once I have seated them 4-5mm into the upright, only 15mm or so will be
left protruding to support the drawers.