In 1996, I put a 2.8l w/ flatop pistons mated to a n-42 head in my 72 Z.  I then transplanted 75 fuel injection onto it.  I got 28mpg highway and the car went like stink.  Summer of 97 I drove the same car from Tacoma, WA to Montgomery, AL.  I spent 3 days driving (Montana was a blast).  It was a round trip affair and all I did was change the oil before and after.  My current Z has a Flattop F-54 block with early E-88 head (thanks Sunbelt), Isky Cam, Triples etc.  I drive it everyday (well almost) and would not hesitate to take it accross country.