Switching to 4.30 gears

Here's a link to an interactive calculator.

http://vexer.com/68rt/speed.html

When you say you want it to top out at 6000 rpm when do you want that? In the 1/8th mile or 1/4 mile? If your running 1/8th mile the 4.30's will work good for you. If your running 1/4 mile their probably a little steep unless the engine is built pretty stout.

If your just talking top speed either should top out at 6000 rpm. It's just that with 3.55's you'll be running about 133 mph at 6k and the 430 gears will be running about 111 at that speed.

BTW: If your running an automatic there's an adjustment called "torque converter efficient". Every torque converter varies but a 95% setting will get you close.