The weight you are referring to is necessary for overall balance of the converter itself to keep it a neutral balance converter.
An external balance converter will have big weights on either side of the drain plug. If this is the case, knock them off if you don't need them and you're golden.
As for the 727 vs 904, I'm sure someone has a more scientific way of telling you the difference, but in my experience, just try to install it. You'll find out very quickly if it works or not. Be careful you know what it feels like for it to be installed on the splines, otherwise I hope someone else chimes in to help better identify the converters.
As far as between small block and big block, no difference other than the flexplate. The same unit stalls differently depending on the power output of the engine in front.
Oh and this should go without saying, always be cautious with unknown converters. If you must use them, try to drain via drain plug and flush with new ATF to prevent any debris and general crud from getting into your trans.
Hope this helps. Good luck:burnout: