I have a 69 camaro and the ends of both frame rails are rotten. my question is what is the best way to fix it. I have two nice frame rails , should I replace the whole rails, or can I just weld in the pieces I need. Whats the best way to go about this repair.Thanks in advance>

replace the whole thing. Rust sucks

Agreed. Think of it as cancer, it will keep on going till it wins.

I also agree. The interior of the remaining frame could be in worse shape than the outer.
It has always been my practice to never weld a repair piece onto an existing frame for a number of reasons. It's more work to install the entire rail, no question, but, in my opinion, that's the only way to go.