A calculation for square tubing will not bring you anything, as the stock frame is 2 u channels welded tgether, and not even all the way through. if you weld it up completely, then you will sort of have a square tubing frame...but still not exactly. Gotta disagree. All frames, whether fully welded or spot welded, have a certain torsional strength value, just "somes better than others". My frame is welded throughout the length of the wheelbase. So I ought to be able to make some comparisons for different segments of the frame.
What do you want with torsion? You're not talking about a driveshaft, considering the whole, yes there's torsion but what you want to look at is just stiffness in a single direction. Gotta disagree again. Pure stiffness is fine for when you're running over speed bumps in a parking lot, but torsional stiffness is what you want during cornering as there are different mass and c/g moment arms, along with different f/r spring rates trying to twist the frame into a spiral.
For a same diameter and wall thickness a square tube will be stiffer than round, however the round will have a significantly less cross sectional area (and weight), if you even out the cross section (larger diameter round tube) the difference will be less.
If weight is no issue and size isn't either, a square and round section, same weight, same wall thickness...the round will be stiffer.
D = OD, d = ID
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Area of round tube = PI/4 (D^2 - d^2)
Area of square tube = (D^2 -d^2)
Moment of inertia, round tube = (PI/64)*(D^4 - d^4)
Moment of inertia, square tube = (1/12)*(D^4 - d^4)