If this coach is a 1979 and is completely stock, it can only have the 400 motor. The 400 was the only engine offering in the full-size Lincolns for 1979.
The 400 was introduced as an optional engine for the 1977 models for the fuel-economy minded, while the 460 remained the standard engine.
For the 1978s, the 400 became standard and the 460 was the optional engine.
For 1979, the 460 wasn't offered at all.
I either own or have at least driven both coaches and normal Lincolns from 1977-1979 with both engines. Personal opinion is that the 460 is overrated. I think the only reason the 460 is the perferred engine amoung collectors is for no reason other than being able to say "I have the 460".
In my own experience, the 460 seems to have more low end torque, afterall, it was also Ford's preferred truck engine, but as far as acceleration, top speed, and performance, I think both the 400 and the 460 are pretty near identical. The one BIG difference I have between the two engines is that the 460 does have a much larger appetite for fuel.