I do not have the answer to your question, however the pressure increase needed to bulge the 20 litre HDPE drumms is low. My concern is if the HDPE has been degraded by sunlight and cracks. Pressure would allow the mixture to be expelled more than just when shaking. It is mainly due to heating of the airspace and added methanol vapour due the the higher temperature of the methanol.
Providing the methanol does not actually boil, I believe that your steel tanks, once the leaks have been fixed, will easily take the pressure.

Regards,
Tony