Heating with an old style wood stove is as much art as it is science.
For a non-airtight stove, a combination of intake and outflow can and
does make a difference in both how hot a fire burns and how long it
burns. Your situation seems to call for an 'every little bit counts'
First install new gaskets around the door seals. Take the ash pan out
and hammer it back into shape for a tighter fit. Install a gasket there
too if you want. Of course a fire needs oxygen, but too much from
directly below will waste the heat by sending it upwards too quickly.
These will give you much better control of the intake.
Then, go ahead and install a damper. Keep it 3/4 to full open while
you build your fire and get it good and going. Once you've got a nice
solid burn, you can ease back on the damper, letting the smoke up but
keeping the heat as low as possible for as long as possible. This
combination Will help. As you learn how you're stove burns in your
house with your wood, you'll find the art of setting it to burn hot all
night (assuming decent fuel). A well operated wood stove is a comfort,
not a chore. Stay warm! Houston