If you're racing the thing and need the car to pitch rotate, a smaller diameter bar, wound up to hold the car at ride height, will help. Good shocks can be used to control the movement. If it's not that serious, I would probably be careful not to get too small with them.

Measuring is the best way to determine what you have. .81 diameter is the drag race only MP bar. .83 is 6cyl, .85 and .87 are 318 & 340 V-8 bars. .89 is a big block bar.

They will get weak and sag. Using scales when adjusting them will help get them right and catch a weak bar.