355's to 391's how much does it help?

i used to street race a 68 barracuda 340 s 4sp with 3:91's. 340s with a 4sp like 3:91s because the ideal shift rpm (6000) for a street 340 comes quickly in first and second with third and fourth being matched well to the combination of 3:91 and the 340's power band. a 340 with headers, a holley 750 dp and 3:91s will outrun any "old school" GTO, Ford, Chevy or big block mopar in the 1/4. however, you DO NOT want to install 3:91s if you are going to be doing a lot of highway driving. 14-15" tires with 3:91s will turn your 340 at right around 3000 rpm at 60 mph. you're probably going to blow up your 340 if you try and drive 70 mph with 3:91s for any length of time. if you are going to do a lot of highway driving, i would stick with the 3:55s. but if you want to smoke the tires in first or second on a regular basis or run your car like you stole it a 1/4 or a mile at a time, you just can't beat (literally) 3:91s.