If you're going for an authentic restoration obviously not. But since you're putting a SBC in it, that must not be the case. I'd say do it, I know I wish I had if only to improve ride quality. It flexes and shakes way more than I would like for a daly driver.
Roughly speaking (someone jump in here if I'm off track) with a 60/40 weight distribution you'll want to focus on a point 40% of the wheelbase back from the front wheels. That will be where most of the twist is happening. I'd also not just box the individual frame rails, but create some "X" braces to tie the tops and bottoms of each side together. This creates the effect of a large torsion tube.
'69 1/2T stepside, Sea Foam Green, 250 L6 w/ 3 on the tree, not an option on it, in the family since new.
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