What is my correct carb??

[Scottcoda]

after replacing the fuel tank, fuel pump, lines and sending unit I
noticed that my rochester started leaking pretty bad from the fuel
bowl. I put a ton of plumper's dope on the nut threads but it just wont
quit leaking, it seems that the larger nut that goes into the fuel bowl
will not seat... it just keeps spinning.. after looking at the threads
in the fuel bowl i decided a new carb might be in order.. the threads
are pretty much shot and a new carb is not a huge investment,... so
after taking the id numbers off the carb I have come to find out that
the carb came off a 73 .. the carb id is as follows 704314 (build code
of 171 3a).. I would rather buy the carb that orginally came with my motor.. so anyone have an idea what should be on a 68 gmc 2500 6cyl
250?? The intake seems to be orginal and the build date on the motor
looks to be the right motor.. just want the proper carb.... help and
suggestions appreciated