i would suggest the 5/16th as a minimum myself. maybe even go to a 3/8th.

next question being how thick is the spring perch material to be tapped to begin with?

i would also suggest for your flat stock to put a bend on each end that will come down around the outer edge of the spring to help clamp it down better.

and just throwing this out there as a in case.... but i assume you know this already, make sure to get the proper size bit to drill for tapping. i have seen on more than one occasion someone wanting to tap a 1/4" hole for example first drill it out with a 1/4" bit and not know why they could not tap it after!

2000 SPORTY, DEF BOY customs, mmm....lockers, mmm... gears, mmm... double transfer case, mmm... stroker