A ladder frame from the 50's is meant to flex. Boxing will stiffen and may cause cracks. I believe 58-59 are a little thicker channel than earlier. Hot Rods, with different suspension than yours, and modern fully redesigned frames are boxed. There are boxed and non boxed opinions all over the place. Bottom line, it will handle your drivetrain. Also, special care must be taken when welding on those frames as is described in the Maintenance manual. On a side note , I have put Big blocks and Small blocks in TF trucks, no problems yet. With the added speed/acceleration you will tend to drive more squirley, you need power steering and power brakes w/disc in front.
