hi
i have an 88 k5 and i think jfab is right how he says,you could put an 88 cylinder on later cases but like i found out if you wanted to drop a later cylinder onto 88 cases you might have issues,the 88 power valve is a diff setup just two valves,as for the 89 on they have 3 parts to the valve,so on 89onwards the case halves have 3 holes to allow for this where as the 88 only have 2 holes,i can see no reason why it wont work either.
sure someone will highlight any other issues,good luck