The 84 and forward through the L98 that is in a 1990 Vette seem to be the same.  1984 is the year that Chev introduced the one piece rear main seal and also introduced  the larger flywheel  Your Vette should have the smaller flywheel or flexplate. ( 153 teeth 12.5" )I did an engine swap not long ago, the replacement came with a 168 tooth 14" +- which caused me much grief untill I understood how everything works. If your new motor comes with a flywheel other than what you have installed originally, the flywheels can be changed out. However when you find a ballance weight on the new motor flexplate?  Find out by looking at both the flexplate or flywheel and the harmonic ballance hub in front of the new motor. Leave the harmonic ballance(r) alone.(Most Chevrolet motors are externally ballanced.) Then if you need to change the flexplate you can have the correct size ballanced to the one removed from the new motor you bought. Machine shops do that sort of work, even electric motor shops should be able to do the work. JRB