I think you're right, Marshall, that it is a 610. It is, however, a 4 cylinder engine. Only the Maxima wagon came with a 6, and that was the same sweet 6 as the concurrent Z used. This one is sort of neat, but I'd want to check out the rust. It looks like the rocker and door bottoms are rusty. Once they start on these cars, it tends to mover quickly.
Pretty sure they didn't have Maximas in the 70's, are you thinking of the 810? Hopefully memory is serving, lol
And before the 810 started production, the six was the engine for the Datsun 710. The 610 in JAPAN could have the L20A 2.0L straight six (along with a stretched front clip to fit it), but all exported models got four cylinder engines, L16/L18/L20B's.