That sounds like a cool solution! Using the adapter below and combining that with your welding idea would get you an awful lot of 1 stage gearing! This would also work with boring out and broaching a cluster gear for a 1/2"hex or finding an off the shelf output gear that matches your intent.
If you can use 2 stages and don't feel like modding anything another idea would be to buy one of these 8mm to 1/2 hex shaft adapters:
http://www.andymark.com/product-p/am-0588.htm
Depending on what you need for gearing you could then use a 50 or 56 gear on the CIM to a 14 tooth or any of the cluster gears. On the output stage keep everything as close to 1:1 as you can, 50:50 may be an ok option. Or adjust as you need, it doesn't really matter because it's modular.
Hope this helps get you going!