The boost valve manually controls the shifting pressures within the tranny. Although you can just replace the stock one with the .490 valve, it would be recommended that you do a shift kit to go along with it.

The installation can be done by just dropping the pan and you can manually adjust your pressures with this by the turn of a screwdriver. Some builders always turn the pressures up a tad upon a performance rebuild, so its always good to know for sure how much it is turned up so that it correlates with the tune.