External BB cranks not spinning well I have a Cannondale F2000 that I'm updating and turning into a gravel/commuter bike.

I replaced the original square taper bottom bracket with a new Hollowtech II version.
Now the new crank does not spin freely at all.

One thing to note, the crankset spindle is too long, but I don't see how this would make a difference to the whole thing.
You can probably make up the extra room for the long crank with spacers, normally 5mm worth for a mtb crank in a 68mm shell.

I would want to find out if the bottom bracket shell has been 'faced', which is more important for external-cup bottom brackets than it was/is for cartridge square taper BBs.

Facing is removing any paint or imperfections on the sides of BB shell, where the cups sit against it. If the bearings of the BB are not perfectly parallel to each other, they will not spin as freely and it's those imperfections that cause a lack of parallelism. 

This isn't so bad with 'cartridge' BBs because the unit was made perfectly parallel in the factory and you just have to thread it into the frame. Being parallel also helps with the wear life of the bearings, one reason cartridge square taper BBs are still the darling of touring cyclists, where reliability is crucial.

This video shows the whole process in detail so will explain better what needs to be done, but it is one of the few jobs that make clear economic sense to have done at a good LBS. The tools are expensive and aren't justified to purchase as a one off, so check whether the BB shell looks like it has been faced, and if not, find a good shop that is able to do this for you.