Just a little piece of trivia for you to consider in a hunt for a tank: I have two ´78 SRs and an ´81 SR. The fuel tank from the ´81 fits on all my bikes slightly better in my opinion. The ´81 tank is slightly narrower where it slides over the two round rubbing bushings at the forward end of the oil tank. I found that because of this, the ´81 tank doesn´t move laterally quite as much as the ´78 tank did.
If you do have any problem though, it could be corrected with a bit of tinkering.