I think it depends on how violent the car is wheelie-ing, how heavy the car is, what type of rear suspension you have, possibly could all be a determining factor... Are you trying to just keep it off the bumper doing 3' high S/S style wheelies or using wheelie bars as an actual suspension tuning aid... I've used a few kit styles from Chris Alston & Chassis Engineering or Competition Engineering, all were on 4 link coil over strut front end cars, just trying to keep the front end from raising to high, wasting forward progress or momentum, all were tubular style, The flat sprung strut style were junk, for what I was doing anyway... I believe length is dictated by the rear of the car, where the chute is mounted, wheel base, HP/TQ, how high the car is going to be up in the air on rear wheels, coil sprung type help to lesson the slam down effect when it goes really high in the air... I like the solid type on my cars better... the longer they are, the more leverage you have {maybe where fuel tank is, type of rear suspension could effect length etc.}... I like adjustable tubular type with a triangulated ladder on both sides, mounted with hiem joints, to brackets welded on the rear axle housing & a Y or X brace in the middle on the bottom ... I would set/adjust them to allow the front end to just get above the height of the staging beams & as long as it didn't get up on the tire too hard & just spin, I would fine tune them, by what direction the car would veer when on the bars hard... most were in the neighborhood of 4'-5' or 6'-7' long, not sure on a 64 B-Body how long that they should be... Kit form aren't much cheaper than already welded bars, shipping was the biggest difference in price IIRC, I never liked the strut type with the flat strut spring on the bottom, too flimsy & bent really easy on my cars anyway... Most all Chassis builders sell/make them... I'm not sure if that helps you any...
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car looks great by the way