Did Boeing or Lockheed ever seriously consider releasing a widebody airliner powered by low-bypass engines?
I am not aware of any manufacturer making a serious proposal to develop a jumbo jet with low bypass-ratio engines. Most airline manufacturers tend to develop a plane from  customers requirements, whether that customer is military or civilian. Civilian airlines in the late 1950s were already nervous about the much higher costs for jet-powered aircraft and there was no call at this time for an even larger plane.

To get the most value for their development efforts, airplane companies tend to look at if they can develop a commercial and military version. As an example, Boeing tries to develop a commercial variant to all of their military planes and vice versa. Starting with the Boeing 307 Stratoliner and continuing with the the Boeing B-52 Stratofortress and the Boeing B-47 Stratojet, Boeing worked to develop a passenger variant as well. There was discussions with making the B-52 into a passenger plane, but commercial airlines were not interested. Some of the reasons were the the hull was very narrow, much of it un-pressurized and in many ways, it  would have been ill-suited for passenger use. The plane would have had identical engines to the B-52. I could not locate any drawings of what it would look like. The push for a passenger plane became the Boeing 367-80, which developed into the Boeing KC-135 Stratotanker and the Boeing 707, which entered into service in 1958.

Lockheed had it's hands full with fulfilling orders for the Constellation. It developed some great military planes, but never pursued a passenger jet until the Lockheed L-1011 TriStar which first flew in 1970.

After World War II, Douglas lost a lot of military business and was forced to scale back. It's first jet that became a passenger jet came out of the development of a tanker for the Air Force that eventually became the Boeing KC-135 tanker. The reason they developed the Douglas DC-8 was that Pan Am ordered 20 of them and they could see the future was jet-powered.

The next big jump in airplane size came out of a development effort by the Air Force. In 1964 they asked Boeing, Douglas, General Dynamics, Lockheed, and Martin Marietta to develop a prototype for the CX-HLS cargo plane. This was much larger than anything that existed at the time. In this proposal, the Air Force was asking for high efficiency engines as well as the ability to carry a lot of weight. The two final prototypes became the Lockheed C-5 Galaxy and the Boeing 747.

By this time, the age of low-bypass-ratio engines was over because of the much better efficiency available in newer high-bypass engines.