Tires/HP/ and lowering!

[0311fastco]

Hello All! I am back from Iraq and started a frame off for my 68 c10, I am going to be running a 454 big block, what would you recommend for the lowering ratio? I am wanting to go with 3/5, will it change with the weight of the motor?
also I am going to be pushing around 600 horse power, give or take and are concerned with catching traction when really getting on it, I have the chris alston 4 link now trying to get a good tire size to grab. My uncle was thinking a 280 or 285 in the rear or do you think that is too big? I am not tubbing Just stock wheel wells I want something that wont rub and want the fenders to just cover the tires Just like Sactojims if you guys have seen his. My truck is going to be used only for weekends and toying around, Thanks for your time and knowlege.