Hi Westcorkrailway, understand your idea both being 4-6-0's but the B12 is by comparison with a Bandon tank quite a large loco. Full marks for lateral thinking! The large wheels and longer wheelbase will be the problem.
The chassis is the hardest part but the SSM kit goes together very well; hand on heart I struggled to work out how to make the brake hangers removeable, when I sussed it I kicked myself! What you need is a Poppys Loco Builder Box to help put the chassis together. Its an adjustable jig that comes flat pack in the post, dead easy to assemble, will help you with any future chassis you buy and is not expensive! Just as usefull as a soldering iron. I bought the one for 3 axles and wish I had bought the 4 axle one. I have no connection with Poppys woodcraft just a happy customer. But which ever route you take, happy modelling.
Mick