Ketchup,
Adding weight at the headshell would make your horizontal eff. mass even higher. Doesn't seem like that's the right direction.
Assuming a horizontal eff. mass of 25 and using the usual formula gives a caluclated horizontal resonance frequency of 8.83 Hz, well within the safety zone.
If horizontal eff. mass is 35 horizontal resonance frequency would drop to 7.26 Hz, a bit lower than optimal.
It doesn't look like a serious mismatch. Lower compliance cartridges might be better matches "on paper", but I doubt you'd have any actual problems with a Bloom.