Oil pump and timing chain questions

Guys, when installing an oil pump in a 340 small block, should there be a gasket between the pump and rear main where it bolts to? Eberg says no gasket or it will blow out. What are your thoughts.
On the timing chain, I've read a lot and am confused. Do I align the gears with the cam at 6 O'clock and the crank at 12, or do they both need to be at 12 and if so, why? Thanks