Stovebolt frames were designed to flex. Trying to stiffen one by boxing, etc. usually results in stress cracks forming at the "reinforcement" points. Compared to the Ford frames from the 1930's and 40's, a stovebolt truck frame looks like a tank, and those frames routinely survived Chrysler hemis, Olds Rocket engines, and Pontiac tri-power 389's. Unless you're planning to drop in a blown funny car engine, you're probably good to go with some well-engineered motor mounts an an added crossmember or two.
Jerry
