I'm sure other more knowledgeable post war experts may have more to add to this, but the most glaring thing I would make sure of is that the insulator between the collector assembly (bracket the 2 center rail rollers are mounted to) and the frame is truly insulating. (and is still there - you didn't accidentally "put aside" any pieces during disassembly, and then forget to put them back, right? )
That and the wire itself that runs from the collector assembly/rollers to the motor. Any missing insulation along that wire could easily short to the frame (especially if the wire accidentally got pinched during re-assembly)
Good luck.
-Dave