From Western airliners only the VC-10 was stongly-built and could be converted to MPA.
Ummm...no? No offense but it's not entirely clear as to what you're talking or asking about.
All Nimrods ever to fly were direct conversions of Comets left on the production line when they quit making them, due to nobody buying them. The P-3 is based off an airliner design, though a very deep re-engineering of such.
There were proposals back around the time frame you're looking at for Western jet-propelled MPA submissions. Boeing and Douglas proposed jets based on existing airliners (737/727 and DC-8/9), completely new designs and something in between (one Douglas proposal was for a tri-jet DC-8/9 hybrid of sorts). I actually found out in doing research for a project that Lockheed proposed and all-new twin jet-based MPA with an unusually wide fuselage in order to accommodate a weapons bay and sensors and other goodies. The US Navy decided it was simplest to go with block upgrades of the P-3, but Lockheed noted that the fuselage was wide enough for a twin-aisle airline configuration and dusted it off for an American Airlines specification, eventually growing a third engine and becoming the L-1011 TriStar.