Here's what the Miller Stability Rule
says about heavy bullets in the slow twist Marlin Microgroove barrel. I assumed the Fryxell nose design, wheelweight alloy, standard atmospheric conditions, and 1800 fps.
300 gr. -- 1.43 stability factor (good)
325 gr. -- 1.23 stability factor (ok)
350 gr. -- 1.03 stability factor (risky)
375 gr. -- 1.00 stability factor (not good)
Less than 1.0 is considered unstable.
1.0 to 1.2 is considered marginally stable.
1.2 to 1.3 is popular for 100/200 yard jacketed benchrest shooting.
1.5+ is popular for hunting rifles.
Conclusion: the Marlin 1:38 twist should be OK up to 325 grains with wheelweight alloy. This is consistent with real world experience.