I'm a bit late to this conversation but like
@Stanleysteamer I was going to say clamp something to the spring rather than weld directly to it. The heat will do weird things to the hardening and tempering of the spring and it might be appt to crack after being in use for a while. If you're drilling holes in springs it'll be interesting to see how your drills perform. They're usually harder than cheap Screwfix special drills. Maybe this is a job for cobalt drills.
Near where I used to live in Birmingham there was a custom phase 2 Standard Vanguard. The rear doors had been welded shut, and the rear springs had been replaced with single leaf ones of the kind you sometimes see on light commercials. They were considerably longer than the originals so the rear shackles sat at a very oblique angle. The shock absorber brackets had been welded to the springs, which I thought was a bit naff. Upfront, some sort of V6 engine had been substituted for the original Standard item, and a coil spring front subframe from a later vehicle had been installed. To accommodate all this the inner wings had been modified, by the look of it with a hammer and chisel. Despite the rather rough and ready engineering, it served as someone's daily driver for several years.