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There are a huge number of differences. L-46 is a high compression 11:1 smallblock that was used in 1969 and 1970. It had a 4 bolt main block, forged pistons, heavy duty rods, an agressive camshaft profile, big valves (2.02/1.60) and was never offered with an auto transmission.
L-48 was the base motor designation from 73 through 1980 or so. It's a much milder beast, though I can't quote you specifics.