The problem with safety data sheets is that they can make you unnecessarily paranoid. Their readers include firemen who fight disasters at chemical plants, so SDS mention dangers that are lethal with 100t or 1000t but negligible in smaller amounts. To anyone who starts reading SDS, I recommend to read also the SDS of ethanol (the alcohol we ingest in wine) and sodium chloride (table salt).
I have nothing against a smaller container nor against the transfer. For heptane, the container can't be polyethylene nor polypropylene, despite these are common, so better check it before.
I have much against the big drum full of air and flammable vapour. At some time, this condition will exist, and any ignition cause will hurt you or worse. So measures to shorten this condition is desirable.
I'd try to put the drum on some stones with its opening at the bottom, to let the remaining liquid flow out. The next step is unclear to me. I'd like to blow nitrogen or CO2 in the tank as long as vapour and liquid traces remain, but probably you aren't equipped for that. Maybe you can rinse with water, and isopropanol emulsifies the heptane (?) but then you have a polluting liquid.