you should be able to pull 1.60's pretty easy w/ 600 rwhp. As mentioned 4 speeds are hard on parts. I'd think you could start w/ what you have and some drag shocks. I'm trying to get down to 1.55 and so far no luck. I might try and clamp the pass side spring next time out. I have XHD springs too w/ rear drag shocks and brand new MT radial Pros (275/60/15)
If I can't get it to 60 foot better I might try the cal-tracs. Right now I'm around 1.60-1.65