Trying to decide what gears would be best for me.

On street tires, will 3.73s be much quicker (1/4 time) than 3.42 in a GM 12 bolt? I wont be running slicks and this car is used to blast around town and down the highway?

Im about to pull the trigger on a 12 bolt and was wanting some feedback from you 12 bolt owners...

Since you have the 383ci. and a 3,600rpm stall Vigilante, and you are running street tires, 3.42's might be to much gear here. If you were going to run 27.5" - 28" street Drag radials, then 3.73's might be a better way to go on the street.

Is there a major diffrence in 3.42s/3.73s since its a 12 bolt instead of the OE 10 bolt? How much traction is added by a 12 bolt?

A 12 bolt does not add traction. A 12 bolt will take more abuse before breaking than a 10 bolt.

"Is there a major diffrence in 3.42s/3.73s since its a 12 bolt instead of the OE 10 bolt?"
are you asking difference between 3.42 and 3.73 OR between 12 bolt and 10 bolt ?

3.73 thru a 12 bolt with street tires

vs

3.42 thru a 12 bolt with street tires

I just wana make the BEST choice. The car will be used to blast around town and thats about it. Mayb a time of 2 down the strip.

"blast around town" = 3.73s' in a 12 bolt