Mountian class loco's

[Bushcreature]

G'Day, are the mechanicals and chassie the same on the light and heavy class mountains loco's? I am trying to get a light for scratch building into  local [ SAR- South Australian Railways ] loco's and seem to only be able to find heavy mountains on the net. Thanks!

[SteamGene]

Since the light can negotiate 18" radius and the heavy cannot, I'd say the chassis is different. 
Gene