to answer one of your questions the silvertop has a cast alloy rocker cover where as the blacktops are plastic.. silvertops are early engines,ranging from early to mid 90's where as the blacktops are made from then on. blacktops have solid lifters where as silvertops are hydraulic. blacktops also have a longer con rod/shorter piston. depending on what you intend to use the engine for blacktops are more readily available and can still be bought new.