Math formula to compare sway bar diameters.
You have a 3/4" bar and you want to compare it to a 1 1/4" bar:
 The 3/4" bar is .75 and the 1 1/4" bar 125. Common denominator for
 the smaller bar is 3,  squared is 9.   Denominator for  larger bar is
 5, squared is 25. The difference is 16. 
   9 divided by 16 = .5625, derives to a 56.25% increase in resistance,
 to each coil spring.
 That would only increase the spring resistance, of one front wheel, at
 a time.  Whereas both front wheels going through the same irregularity, 
i.e.,  a speed bump, would not employ the resistance of the bar. 
 (Buddymander  SDC FORUM - Dec 07)

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The formula (seems) sound, but there are some factors that also
will effect the bar strength - like the length of the torsion area, and
also the lengths of the "arms" that transfer the torsional force. If
the arms are longer, they will have more leverage and less strength.
This is one reason the Impala SS guys will use a 2nd Camaro SS bar on
the front. The arms of the Camaro bar are about 1/4 to 1/2 inch less
in length - which tranlates to a flatter road race car. The bars are
the same diameter. (SBCA96 - SDC FORUM - Dec 07)

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Using a full length early lark V8 bar that attaches only to the lower control
 arms, and a late lark V8 bar that you will cut off end sections that would
 normally mount to the lower control arm. With them stacked on top of each
 other, weld together in the area just ahead of where the early bar mounds 
to the lower arm. Finally, be sure to weld on both sides with good penetration.
 You will still use the frame stand-off mount, but for added strength, you 
will need to "box" it in. Urethane bushings are also reccomended here. For
 springs I used 1964 Cadillac Fleetwood springs with 3 coils removed. Trial 
and error..cut, install and measure,etc,etc. The Caddy coils were correct in
 outside dismeter, the big differens is the "wire diameter" is much larger. 
Upper and lower control arms have Delrin(sp) bushings with the upper shaft 
flipped for maximum effect. In the rear, the stock V8 leaf springs have an 
extra middle leaf added for stiffness, NOT added height. I used a stock rear
 bar from a 57' President sedan, again, with urethane bushings. The shocks 
are Gabriel Adjust-o-Matics on Xtra-firm. Remember guys....you have most of 
your weight up front...that's where you need the heavy duty stuff. If you set
 up TOO stiff in the rear, you'll have a real bear to handle. Trial & error
 depending on the model your're puttin' together. I built this car in 81' and
 so far it's been a blast hang it out when you want, it won't when you don't
 want. Also known as steering with your right foot. Hope this will be of some
 help.
(Dan Miller "ROADRACEARK" - SDC FORUM - Dec 07)