There are (at least) four distinctive refinements to "an engine known as 318"
In some ways they can be though of as completely separate engines
Poly 318--what you have. You cannot use LA or Magnum heads. Some other parts such as crank may fit
LA 318--the later 318, an outgrowth of the 273. This includes the 273, 318LA, 340, and 360
Late 80"s ?? so called "roller block" a late LA 318 or 360 which came with a factory roller lifter cam
So called "Magnum" engines. These (318 and 360) use different HEADS than the 318 LA. You cannot just toss magnum heads onto an LA, as the LA oils the rockers through oil holes up through the block, heads, and rocker shafts, while the magnums oil each pushrod and each rocker through the pushrods. They do not use rocker shafts, but rather 'stud' rockers more like Chivy/ Ford
In addition, the angle of the intake manifold is different between LA and Magnum heads.