The B&M® Shift Improver kits™ feature two options to recalibrating your 1968-1981 GM TH350 automatic transmissions. First is the HEAVY DUTY set-up, giving you firm positive shifts without loss of driver comfort for improved performance in all street vehicles. This is a great option for performance street driving or towing. The second is our STREET/STRIP set-up. Be warned, this will deliver firm positive shifts you will feel, along with easily chirping the tires when shifting somewhat aggressively. This setting is recommend for high performance street/strip use, or for competitive events when you want the hardest, quickest shifts possible from a shift improver kit. Note: Does not fit GM TH350C with lock-up. Use part# 30235 for TH350C transmissions. Fits 1968-1981 GM TH350 and M38 automatic transmissions. Does not fit TH350C lockup (use part# 30235 for TH350C)