frame for 63

what size of square stock do i need ..i am building a backhalf for a 63 impala 4 door

man i would use 2x3 or 2x4 3/16 my notch is 3x3 x3/16 but these frames are pretty flimsy so a back half would be nice but i would recomend reinforcing the x in your frame while you are doing so  i will be doing that very soon

i used round notches and reinforced the stock frame on my 63 impala 2 door

just used some flat bar and capped off the rear frame rails

I definitely agree, I used flat steel on our 63 SS and welded it solid, basically boxed it.  Made the whole frame almost 75% more solid. Also i went through the rest of the frame and re welded all of the areas, especially under the drive shaft tunnel and on the sides, they are notorious for cracking. Also if you notice the frame is only welded about ever 6 inches for an inch or two.