The 3.8 in the 1991 Sable my parents used to have was a good engine, although we only put 89K on the car before we traded it in. I'd say look out for the service department at the Ford dealer more than look out for the engine. The original plugs went over 80,000 miles thanks to our wonderfully honest dealer (who sold us an extended warranty then when we had to replace the ignition switch told us it wasn't covered because it's not a manually-operated switch).
Had they switched the T-bird from the 302 to the modular 4.6 yet in 1994? In any event, the 4.6 might be a good engine, but a downside of it is that it's as big on the outside as a Boss 429 but has slightly less displacement than a 289.