Yes, most Z28's got their engines replaced (and sometimes trans and rears) very early in their life; oftentimes because young drivers not familiar with what happened when you over-extended the rpms on a solid lifter engine!
I had several friends with Z28s in the '69-72 time frame while I was in the USAF (I spent all my free time in the base hobby shop working on my cars - '60 Chevrolet Impala, then a new '70 RR, and during the '71-72 timeframe I bought a solid Henry J body and put first a 283 /PG combo in it, and then in '72 due to the influence of my buddy z28 racers, I bought a '67 302 from a '67 Z drag car and installed that along with Muncie M21 in the Henry J)... During that time, one of my friends with an orange/white '69 z28 went thru several different rear gear sets trying to beat the local street racers.. from the 3.73 to a 4.11, then to 4.56, 4.88, and finally 5.38 gears!)..
The fast fella in the area at that time worked in the local speed shop and had a '70 Nova 396/L78, 4speed that was drastically enhanced!)..
After I was discharged in '72 and completed my engineering degree in '74, I bought a marina blue/white '68 Z28 as my play car ( no significant options but had the original drivetrain which ran like a scalded dog!). Two years later I traded that to a policeman who had just purchased the '69 I own now. The '69 had all original components as well but the engine needed a rebuild; the advantage to me wtih the '69 was that it had many and some unusual options which I considered a step up in collector value over the 'base '68). I rebuilt the engine, and drove the car for 4 yrs before garaging it for a 'some year' restoration, which I later decided would have been a mistake due to the originality of the car!
Anyway, William is correct about *most* Z28's even in the early/mid '70's having their mechanicals 'long gone'; most of the Z28s advertised for sale during those years were also 'in name only' (ie. had the badges but not the bones).
