have you read through the s10 swap FAQ?
http://67-72chevytrucks.com/vboard/s...d.php?t=632686
building mounts or buying mounts depends on what kind of skill level and tools you have. buying a mount kit you are trading money for time. I have done 10 swaps and building mounts never takes more than about 4-5 hours from scratch for all the mounts needed. Skymangs has drawings in the S10 swap thread for the height you want to build them, you can look up a thread named Albert by Hussey (type Albert and Hussey into the search bar) and he has another set of drawings for mounts.
if you choose to buy, things are figured out for you that would otherwise need some thoughtful planning, like bumper mounts and running boards and bed. its not hard to do those yourself, but a lot of people stall there and a mount kit would be helpful. more on that in a second.
remember that mounting the body is literally the easy part of the swap. everything else, pedals, fuel lines, brake lines, wiring, etc. that is where you are going to spend the bulk of the time. I can mount a cab in a weekend, front clip and bed and bumpers in a couple more days. the rest takes about 3 months of evenings and weekends.
with a 6.0 and 4L80s you will need a new oil pan (camaro or holley) to lower the motor or it will be 3" higher in the chassis, and you would need a big trans tunnel and probably a driveshaft tunnel at your mount height. a 4l80e is also bigger than a 4l60e, so you will need more trans tunnel. here is what 3.5" mounts look like on a 4l60e with the trans pan just higher than the framerail. "setback" depends on your engine mounting height, if you raise it for a truck pan the engine will be more forward than if you mount it lower, unless you tub the firewall.
yours will be bigger, but not a lot. again you can search for OUTLAWDRIFTER in the search field and his project has a 6.0 4l80e.
I mount the s10 booster on the firewall, putting it under the floor will need a kit, a kit that may not work well with a 4l80 trans, make sure it works. steering is done with new shaft and joint, and the exact part list depends on if you center the column or put it in the factory divot, depends on which column you are using, and depends on where that column exits the firewall.
so back to the buy vs build, there are a lot of things that depend on other things. if you are a reasonably competent mechanic and understand mechanical things, you will sail through and probably wont need parts. if you like more "tab a-slott b" kind of assembly, you will have an easier time but a relatively empty wallet. neither is better than the other, just take an honest assessment of your skills and your tools and your available space.
if you need help ask, we like seeing trucks back on the road, no matter how long it takes.