Your welder is correct. Position plates in desired location, tape it, position clutches, you have to play for best angles, mark clutches holes location, see where you can position plates mounting holes.
I have used four, 5/16 SS flat head bolts, countersink them flush with plates, under clutches, so they are not visible. If memory serves me well, I was able to use two or three existing holes, just enlarged them, cause they were 1/4" and slightly corroded, hence 5/16. Yes, aluminum plate inside, not sure of size thou... 1/4" for sure.