I have no way to measure inductance.

To know what size Cap. you will need, you will have to have some idea of the
inductance. One easy way to check the inductance uses an AC voltmeter a
variable resistor (pot), and a low voltage 60 hz voltage source like a 120/12
volt transformer.
Hook the inductance and the variable resistor in series. Apply the 60 hz
source across the series combination. Adjust the pot until the voltage dropped
across the pot and the inductance are equal. Read the value of the pot, this
is now equal to the inductive reactance of the coil Xl plus that 18k DC
resistance you said you measured. From this you should be able to calculate
the approximate inductance of the coil, and the Capacitor value needed to tune
it to 8 hz.
Use L=(Xl+18k)/2*pi*60
and then 8^2=1/4*pi^2*L*C and solve for C.
73 Gary N4AST