Use the search link at the top of this page -- there are several posts giving detailed examples on how to port one device config to another. But usually both devices should be on the same firmware for a smooth port.
The 60D looks to have 2 extra internal ports over the 60B but otherwise I am guessing the ports (interfaces) are named (internally) similarly, so it could have been easy (with minor editing) to port the 60B config over. Unfortunately the 60B is not supported on the 5.0.x firmware and the 60D looks to be a 5.0.x firmware only device. So not sure how well using the current porting methods will work.
An alternate method would be to side-port the 60B config to another fgt model that is supported on 5.0.x, such as the 80C/80CM. The 80C/CM should start out on the same firmware as the 60B then upgraded (eventually) to 4.0 MR3 Patch 14 then to 5.0.x. From that point it should be easier to port this config to the 60D.
Edit: of course the 60B config will have to be edited before it can be installed on the 80C/CM.
In any event, it is advisable that the config on the 60B should not contain any firewall objects/labels with space chars or non-standard char codes. These should be " fixed up" first before the config is put through a firmware upgrade process.
NSE4/FMG-VM64/FortiAnalyzer-VM/6.0 (FWF30E/FW92D/FGT200D/FGT101E/FGT81E)/ FAP220B/221C