The diffs will probably go with the bigger wheels, that's my main concern if I end up converting it... but with yours you are adding the extra weight of an engine onto the chassis as well ( I'm not sure that it will effect it though). The Ofna extenders will work with the hyper st wheels and any other 17mm wheels. Another set of extenders you might want to look at is the pro-line ones. They enable you to use there pro-line 40 series monster truck wheels, there much bigger... but the extenders are much more expensive.
Here's a link to the pro-line extenders.
http://www.modelsport.co.uk/?CallFun...n&ItemID=29221