I normalized using your 21 runs/wicket compared to about 30 for the modern day. Giving me 69.16 and 23.39. That's for an FC record though.
Well first I’m not sure where you’ve got 30 from since I did say it was over a 20 year period. Since you simply asked runs per wicket I did that using the overall/aggregate tab, which gave me 21.18 for then and 32.56 for the past 20 years. (we’ll also remember thats actually only half Grace’s peak and prior to that the numbers would’ve been even lower)
So obviously we’re using different numbers in the first place - I divided his average in each discipline by 21.18 then multiplied by 32.56
But lets go even deeper using the actual batting and bowling aggregates since they slightly differ.
This gives us from 77-87 an actual batting average of 20.07, and bowling of 20.90.
We can either use the past 10 (because thats the sample size we’re using for the tests til the end of Grace’s peak) or 20 (because that’s how long the peak you asked for was)
So average batting for the last 10/20 years 29.22/30.88 and average bowling 30.41/32.33
So extrapolating from here we either get…
(10 years)
Batting 70.48
Bowling 23.82
(20 years)
Batting 74.48
Bowling 25.32
Take your pick.
For reference using the same method for Bradman (batting average during his career 31.85) you’d get 91.69 for the last 10 years (though doing that makes little sense for him) and 95.48 for the last 20 years (very close to his FC average).
Multiplied by .8 for batting and 1.25 for bowling to get something akin to a Test standard average of 55.33 and 29.23.
That makes no sense, sometimes players will have better test stats than FC stats - Bradman, Sobers (in batting), Smith, Hobbs, Richards, Warne, Steyn, and sometimes worse - Tendulkar, most of the rest of the very top tier bowlers.
Seems as though bowling is more likely to be worse and batting better. Though that’s far from a hardline rule.
Many factors at play there - quality of opposition, number of matches played in/out of peak, difference between domestic and international pitches.