Here is my take as I have a "mid travel" k5 and am familiar with the prerunners here in mi. I would suggest starting from scratch, you will have to modify the frame both front and rear to keep the height down. I am already way taller than I wanted, and whats keeping me that high is the stock frame and solid axle. Do you plan on beams or A-arms? Trailing arm rear can be done, but you lose space in the rear seat. I'll be doing a short course style link in the rear of mine next year. Hope to get 15-16" of travel out of my 12" coil overs. Honestly a k5 really isn't a good vehicle for pure go fast as the space for packaging and wheel base make it limited. Can you have fun in one, yes. I blast dunes with deuling at 60-65MPH just fine, but I can't keep up with the guys going 70 plus through 2-3 footers. If I were to start over again I would start with a 2000 ish Chevy 1500. They have plenty of kits to make them go fast and they already have an LS sitting in them. A crew cab short bed, or extended cab short bed would be my ideal candidate for go fast if I were to build. If not I'd buy a full tube race truck, as you can pick up used ones for about 25-30k ready to run.
