I have to agree. While Mouthpieces are sometimes wonderful for broadening your sound's color spectrum, I believe that if your clarinet has not been kept up in the 40 years since it was first played, it might be time for a new one. I don't know what brand your mouthpiece or clarinet is...so my advice is to go to a music store and try a few different mouthpieces (Vandoren B45 is a great mouthpiece that will last a long long time for intermediate to amateur professional players). the good thing about the vandoren B45 is that it is so popular that most music stores stock them, and you might be able to try it out before you buy. It runs about $60 from
http://www.music123.com . You might also want to replace your ligature. I suggest a Rovner Reversible leather ligature. That is $18 at
http://www.muncywinds.com . I'm not saying that it's cheap to replace these, but less costly than replacing a clarinet. You can get a "quality" student clarinet for around $350-$400. I wish you luck, and hope that everything works out for you.
-Dennis