will a 1988 trans fit a 1990 f-350

If the engines and the tranny's are they same ... Yes they should, you might want to double check the numbers on both tranny's to make sure they are the same, both trucks are computer controlled, so that is important to do, If the engines are different sizes, the "bell-housing" might be the same so the tranny will bolt up. But that is all just a guess, You didn't mention the drivetrain in the 88 and the one in the 90. Best of luck.