use top clamp plate as a jig, drill 2 new holes with 8.3mm drill about 3/4" deep tap them 3/8 x 24unf and jobs done, space the plate higher then there is more vertical control of your free hand drilling
the diff you have had a swing spring box which only has 4 studs ,
I did this on my Vitesse with a battery drill ,, check the depths of the old 4 holes im sure they dont go through to the diff, so drilling doesnt put any swarf in the carrier , well my 2 holes didnt
some would say 4 studs is sufficient but on a fixed spring there is more roll forces to contain. so 6 is safest.