i have welded slots for years. it's easy if one clamps a piece of flat copper under the plate before welding. the weld doesn't stick to the copper, so after a quick clean up of the slots to match the length you want, you are good to go.
i did buy one of those plates, but have yet to try it.
my biggest hurdle in factory distributor modification, is getting the shaft retaining clip out from under the rotor seat.
i finally made a tool for that by welding a couple of pieces of tiny tubing that fit the wire size of the retaining clip onto a pair of small, seldom used pliers. that works great !