I know what you mean. When I did my SAS back in '03, I didn't have a welder at home, only the ones where I work, but I couldn't leave the truck parked in the shop while I was doing all the cutting, grinding, and bolting on the new stuff. So I did all the welding over a few days, first the shackle tubes then the front hanger. Then I borrowed a torch and parked the truck in my driveway at home and started cutting. It was a big gamble, as if anything went wrong I would have been screwed. But it all worked out, thanks to doing lots of homework, reading all I could, measuring, etc. My SAS was a home-brew, not a kit.
I personally would not use a plasma cutter to do the shackle holes. It will be difficult to keep them round, and there is some stiffener plate inside the frame at that point that will be hard to reach with a plasma nozzle. Those things fling lots of spatter out the other side of the cut, I'd be worried about catching something on fire.
I drilled my holes with a hole saw. I started with a long 1/4" drill bit that I carefully lined up to keep it straight across to the opposite side. Once I had the 1/4" pilot hole drilled, I stuck a long piece of 1/4" rod in the hole saw, and very slowly started cutting away. Lots of cutting fluid, slow speed, hold on tight to the drill (get a real drill, not a cordless one), be patient, it worked great. Very nice neat hole in the frame.
Unfortunately I can't remember the size of the hole saw I used, but you can just match up to the OD of the tube you have, you want it just a little big bigger.
I should add that I have a 1" body lift, that gave me clearer access to fit the drill in.