Petcock Question

[bellison80]

So I am looking at petcocks on Ebay and I was wondering if one from a 78 cb750 would fit my 79 cb750f. They are almost half the price and the only visible difference would be the routing of the fuel line which would almost work better. Has anyone ever run into this or had any experience with this. The current one is leaking bad and riveted together. I am not opposed to trying to drill it out but is a suitable replacement could be found for under fifty bucks I would be cool with that as well. I also found a non OEM one for less than fifty but states that the bung needs to be 18mm. Does anyone know if this coincides with a 79 cb750f tank?