I would not personally worry too much about the weight... I'd worry about the what this was balanced for and how it would balance with the new parts I was going to use with it.
BTW, OP, are there holes inside in the 1-2 and 7-8 crank throws, drilled parallel to the axis of the crank and from the front on 1-2 and the rear on 7-8? That is a 340 crank. I have to wonder if this is a 318 crank modded to be used with 340 parts.
