Yes, the 60's & 70's GM/Delco radios were designed for use with 8 to 10Ω speakers and should not be used with the 4Ω speakers that are common nowadays.
It's also important not to operate these older Delco radios without a speaker attached. Due to the way the output transistor is connected to the output transformer, it can see some dangerous inductive feedback when operated without a speaker. Connecting the Delco radio's output to an amplifier with a high imput impedance is similar to operating it without a speaker ... not good. I suppose you could put a resistor in there to provide the necessary load and still connect it to an amp.
Fortunately, you can purchase new 10Ω speakers that will work with your original radio (and fit the stock dash location) from places like S&M Electro-Tech -->
http://www.turnswitch.com/speakers.htm I used one of their 4x10 speakers in my 74 Nova and it actually sounds pretty good ... for a 40 year old radio anyway.
Another option would be to use a radio from a slightly newer GM car or truck. It's my understanding that the 79 & newer Delco electronic tuning (ETR) stereos have amplifiers that can safely drive 4Ω speakers. Here's an example -->
http://67-72chevytrucks.com/vboard/s...79&postcount=5 I put a stereo like that into Dad's 77 GMC and it was a direct fit (no dash cutting necessary). I did have to add some wiring because the 78-up radios use different plugs and the ETR units need a couple extra wires for the clock/memory and a display dimmer signal.