The V8 K frame is not any stronger than the slant, which it seems like you might believe. The difference is the engine mounts. You can get conversion mounts and keep the existing K or swap to a V8 K if not already done. I prefer to swap them since its not hard to do and they are relatively easy to come by still.
Disc brakes on the car are a plus, power disc even better. I have seen plenty of drum brakes on 340 cars. Rear end is likely a 7 1/4 unless someone special ordered or someone already swapped. Again, 8 3/4 is a big plus and even an 8 1/4 is not a bad rear for light duty street car.
You really have to take a good hard look at the car and see what it has. If things were swapped, see if they were done correctly or just cobbled together.
Rust is always the main concern. Everything else can be tailored to your wants.
