I paid a pro to build my 8.5, some special tools are required to do the job. It's not like you couldn't buy some tools and learn to do it yourself, but I consider it to be cheap insurance to have an experienced guy do the work to make sure the gears don't make noise or wear prematurely.
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A 3.42 gear is not a good highway gear, especially for a 455.
I would consider 3.23s as the highest gear to consider, with 3.08s being just about a perfect all-around gear for a 455 for street and highway. Running a tall tire will help take the sting out of running the 3.23 gears if you decide to go that way.
I picked the 3.42 gears for my 8.5 rear only because I will be running an overdrive auto trans with a lockup converter, the final drive ratio will be 2.394 in overdrive.
