Blacktops got bigger inlet valves (I think) plus better porting but the mains caps are not as beefy as the Silvertop so the best combo is Silvertop bottom end with Blacktop cyl head. This leads to loads of complications with belt tensioners and water pumps and alternators and engine mounts and exhaust manifolds etc etc etc. Then there's the ZVH if you want to turbo the Zetec. The choices are unlimited but the Zetec E engine is a good 'un. Hard to get past 200HP without spending a lot but pretty tough and oil tight.
Barry