If the front cover is not centered on the crank then the front seal will always leak. Putting a sleeve on the pulley shaft will increase the compression on the seal lips and may help the seal but the seal will eventually wear with the increased pressure.
Even if the front cover is centered when assembling, it tends to creep overtime with the differential thermal expansion between the block and the aluminium cover and all the bolt loads especially those pulling up to the head
Shaft vibration especially at high revs in a racing engine and crankcase pressure also tend to push oil out the seal.
I can build an engine that will stay oil tight in road use at this front seal for a reasonable time but with race use that time becomes very short
Keith of sidedraft_central fame and his obsession to build a twin cam that runs on carbs but feels as good as a modern fuel injection system and does not leak oil like a modern Japanese car has made a frame to hold the front cover centered on the crank despite the loads and thermal expansion issues to help stop the front seal leaks
cheers
Rohan