repairing front frame rails 65 cuda

If that much of it's gone, replace the whole rail and do it right. I repair holes in major support steel all the time, but if the holes connect and it's the length and placement you said - it's not repairable. It's junk and unsafe. It will take a while, but either section in a solif frame rail from the K back, or replace the whole thing. If the car is properly supported and level you will not have flex. Jackstands under the axle and control arms is not properly supported for major frame repair work. You should have the stands under the rear of the frame rails, the front of the rear frame rails, the rockers/pinchweld, and the front of the front frame rails. Then you can remove what you need to and there's enough support for the rest to not twist or sag.