SA-350 Gr.LF2 is generally used for low temperature applications for forgings for piping flanges (eg.ASME B 16.5)where notch toughness (Charpy Impact) testing is required. SA-765 Gr.2 is similar in content to SA-350 Gr.LF2, except that this specification is used for components other than piping flanges. Thus, for a tubesheet, the correct specification in this case would be SA-765 Gr.2. I do not have a copy of ASME II Part A at hand, to check the specific requirements of SA-765 Gr.2, you will need to refer to ASME II, Part A for these specifics.
John