When the head does not seat down far enough on the barrel to compress the head gasket, then the spigot on the barrel is too tall, and is preventing proper seating of the head. This will always cause oil leaks from the head gasket area, at the pushrod tubes.
The proper method is to have the barrel spigot shortened by the necessary amount to let the head gasket be compressed to seal.
Some do this by hand with a file, but care should be taken to keep the top square with the bore, so that it mates evenly all around inside the head for compression sealing.
Obviously this would be best to do with the barrel off the bike, so that no metal filings get into the engine while it's being done.
Whenever possible, it is better to measure the gap with a feeler gauge and measure the head gasket thickness, and have the job done at a machinist, so that it can squeeze the gasket down to half it's original thickness.