A converter will make some difference for sure, but how much difference depends on which stall you use. Ls1's are kinda doggy in the bottom end, so you want to get it up there where it can make some power. Which converter is the best for you would all depend on your setup. With a basically stock setup, 3200-3400 is plenty. Which str you want is really up to you... the higher you get, the harder it hits. A lower str will hit softer, and will work great with nitrous. But not all converters are universal, even at the same size and str, they will NOT be exactly the same. Some of them tend to flash lower than what they are advertised at, so you should definitely do some research on different companies before you buy one.
I could probably type for 2 days on which one is the best, but it's really a matter of goal and price.