[dsraven]

the video is pretty good. that is what I would do personally. I would also add the horizontal reference line on the side of the frame on both sides, either paint the frame quickly in that area and draw a line on the new clean spot before cutting or use tape and a marker or whatever. that way the line can be used to help ensure the frame goes back together flat. not saying it shouldn't also be checked with a straight edge as well.
moving the rear spring perches adds some other issues like centering the axle in the bump of the frame. this can be an issue later if you, or anyone after you, wants to lower the truck. also the park brake cable length issues, etc. it is just better to do what the factory does and simply move the 2 parts of the frame closer together. it would probably be the same amount of work anyway. one part slides inside the other so it is just as strong as original. grind off the old weld though to ensure you get nice clean metal to weld back together and you are not welding over a weld. it is also a good idea to mark the cross members at the centre point between the frame rails and then use a laser (they are getting cheaper all the time) or a string line to ensure the centreline stays true, front to rear. you can go to gmupfitter and download a copy of frame blueprint so you can see where the factory reference points are to measure off, as well as a bunch of other specs. this is for a lot of gm products, not just an s10. a good site for specs if your truck is actually listed (S10 is for sure because I looked at that swap for my 57).
good luck. do the research before the zip disc comes out, haha.
get out the blocking and vice grips, send the wife out shopping and turn off the phone. no interuptions that way. take pics along the way showing detail so you can reference them if this is a project over time.

just as a side bar, you can use jack stands or blocking or whatever to hold the frame up and I have found that those little wooden shim packs from the building supplies store come in handy for the final shim to get everything perfect before tack welding. body washers work pretty good as well because they are thin and larger diameter,but tend to fall out a lot if something is moved slightly. I ended up using some jack stands that I took the moving part out of and just replaced that with a large piece of threaded rod so the height could be adjusted infinitely. I welded a 3/8 bolt to the end of the threaded rod so it would be able to go into a hole in the frame and be locked down with a nut. it worked great because the frame could be set to level perfectly. i have a laser level that shows to the tenth of a degree and it was cheap, like 30 bucks. I use it all the time and have noticed that the bubble level attached can look level but the digital readout says we aint there yet by several degrees. it was an eye opener at first and now I just use the bubble to get close then rely on the digital part to get dialed in. once you have one you will find you default to that all the time. it is good to get the wheels off and the frame on stands and sitting level front to rear and side to side. less chance of a problem down the road. weld the areas in short lengths so the frame doesn't warp from too much heat in a concentrated area. pick different spots to weld while waiting for the last weld to cool. go to the other side frame rail while waiting etc.
have fun,
dennis