1977-1984 Olds 98 and Delta 88 are the same car mechanically. The B/C body wagons (i.e. Olds Custom Cruiser, Buick Estate Wagon and Pontiac Safari) are the same all the way to 1990.
That being said, you should be fine ordering a brake line kit for a 1982 98 for your 1984 98. Even if it's for a Chevy Impala it should fit your 98. The difference is going to be in the wheelbase, as you mentioned. Make sure it's not a brake line kit for a station wagon or coupe and you should be fine.
BTW, that's a mighty fine automobile you've got there!