There is probably an over-lap date of manufacture from Gen 1 to Gen 2. If you've got an early Gen 2 truck 'built' in 86 it'll have a different configuration. If you are either curious or desperate I'd go to a JY and get a look at a Gen 2 tank. Tape measure and phone in hand to record dimensions and reference pics. Can't say it's 'impossible' to modify a Gen 2 tank to fit, but a nut and bolt swap saves headaches...