just because it's free, doesn't mean it automatically a good idea. the 2.8 may share its transmission pattern, but shares little else with the 2.5. is it complete with exhaust, accessories, carb, wiring, etc?
try the classifieds here or try car-part.com for junkyard searches. the 2.5 has remained pretty much the same for 20 years so there's gotta be a decent head out there for ya.
out of curiosity, what precisely has gone wrong with the 2.5?