The battery should have a lip or ledge on the backside of the housing where it fits under a metal "lip-clamp" as part of the car. The front of the battery shoild also have a small lip formed as part of the housing...it gets clamped down in one of three, longitudinally spaced holes, using a small metal "wedge" tab with a hole in its center for the bolt. Usually, the biggest size ( stock) hole is the furthest out. Smaller batteries use one of the two holes further back.
A search will definitely find some recommendations on size and other things like Optima and Odyssey installs.
Wil
EDIT-- here's a start --->
Battery Replacement in a 86 - Advice Please