Lots of RTV  
 
Mine only leaks a little bit - it takes a few times.  Put together and run it - see where it leaks and remember to put more goo there next time.  After 3 rebuilds I've got mine pretty dry.  
You should have seen it the first time I put the Donovan together.  I'm running external oilers to the heads and didn't seal of the bottoms of the head studs well enough.  I must of pumped a quart of oil out onto the floor just idling in the shop for about 30 seconds  
  