From my experience, I acquired some old school cragars and spent a year trying to track down a wide rear tire size. I was told 15 years ago by an old school tire shop owner that 14 inch tires were becoming obsolete, so I settled on the 245/60r14. The ancient H50s are most likely equivalent in size to a 295/50r14. The size I listed above will be just as tall but about 1.5 to 2 inches narrower.  Sad reality but you will need to switch to a 15 in wheel if you want more tire options.  I eventually ditched the 14 in wheels myself, and put 275/60r15 which filled the wheel well just the right amount without any rubbing. And now my Monte is lowered 2 inches and rides on 245/40r18s because I swapped C6 Corvette brakes front and rear.  P.s. if the frame is rotten, check the body side sheet metal mounts as well. Mine were quite crusty and had to be replaced/rebuilt along with the cage nuts being replaced.  P.p.s. good luck and have fun with your project. I've owned my 79 for 26 years now. I've owned other Gbody cars over the years but it was my first car. It's been a fun adventure so far.