From the Electro-Tech website, at
http://www.turnswitch.com/speakers.htm :
Note: Many replacement speakers now sold by the discount stores, auto parts stores and mail order restoration parts houses are labeled as "4 to 8 ohm compatible" and are in fact 4 ohm speakers. These speakers can damage the older transistor radios. 4 ohm speakers will measure around 3.2 ohms with a DC ohmmeter. An 8 - 10 ohm speaker will measure around 7.4 ohms with a DC ohmmeter. The DC resistance is the critical number for the Delco radios as the output transistor is in series with the speaker. A properly operating Delco radio should have 1.5 volts DC across the speaker voice coil. A 4 ohm speaker will draw twice the current of an 8 ohm and cause the amplifier transistor (the half dollar size device mounted to the black finned heat sink on the back of a Delco radio) to run very hot. The speaker cone should be displaced in an upward (away from the magnet) direction about 1/8" when a Delco radio is turned on. If the cone moves down toward the magnet, the speaker leads need to be reversed.
I think the impedance of speaker you have is close enough that you will be okay, but if you do want to use a resistor in series it doesn't need to be that big since the voltage is 1.5V at the speaker output and not 12V. Volts / ohms = amps, 1.5 volts / 7 ohms = .21 amps. Amps x amps x ohms = watts, .21 x .21 x 3 = .13 watts.