Where would you join frames together?

[zapper]

Hi I need some opinions. The frame on the 64 that I bought is rotten on both rails right were the coils mount to the frame, the coil brackets are gone and the frame has holes right through. By the way its a short box. I found a long box frame same year in good condition, I am trying to figure out the best place to cut both frames and rejoin them. Should I do it just as the frame starts going up at the back in front of the coils by the cab mount, or should i cut in front of or behind the driveshaft crossmember loop? Is this a safe alteration? If it was butt welded together without plating it would it be safe or should I box or put a plate on the inside where the welt joint is? Need advice.
Thanks
Ang