khalidkhattak,
While API 510 does not specifically address your situation, it states, "When repairs or alteratiosn have to be performed, the applicable requirements of the ASME Code, the codes to which the vessels were built, or other specific pressure vessel rating codes shall be followed."
Whereas the SA-182 material is permitted by ASME Section VIII, you may use it to plug the tubes, regardless if it is currently in the condition of a blind flange or bar stock.
As metengr stated your best bet is using SA-182 UNS S31803 forged bar stock and machining the tapered plugs therefrom.