Bellhousing alignment isn't really that critical for most stock applications, but can be important when dealing with aftermarket transmissions. I'm installing a Tremec TKX transmission behind my SBF, and Tremec requires the bellhousing opening to be aligned within 0.005" of the crankshaft centerline. This can be accomplished by using a dial indicator to determine the alignment, and then correcting an out-of-tolerance condition with offset dowel pins.