308's were made from '89-91, but they came on 360's not 318's.
But here's the thing, if you're building a 318 they may not be what you're looking for. 308's are open chamber heads (just like the J's), usually with a chamber of 65-72 cc's. They do flow better on the exhaust side than the J's or X's, but not enough really to warrant changing over from a set of J's if you already have them. Better to spend the money having a little clean up work done on the ports of the J's rather than starting over.
302's were made from '85-89. They came on 318's, and a are a closed chamber head (usually around 62cc's). They have the redesigned exhaust port, but they also have the smaller 318 ports. Better quench, but if you're building a high powered 318 the ports will be on the small side. Depends on your build, if you're only looking for 1hp per cube or so they'll work fine with larger valves and keep your port velocities up.
Magnum heads are the best of both worlds, they have a closed chamber design with the larger 360 ports, and flow well for an OE head. Not the best for port work, but if your intent is to make <400 hp they work good in stock form and come with 1.92/1.62 valves, so they can save you some money there over having a set of 308's or 302's opened up. Of course, they do require you to run a magnum intake, rocker set up, and oil through lifters and pushrods on an LA engine.
Next step would be the Indy/RHS X heads. For the price they're awesome, cheaper than doing a Stage 2 or 3 port on a set of J's or 308's, and pretty close to as much flow straight out of the box. But probably not necessary unless you're looking for around 400hp or more, and if you're building a 318 that would be a pretty hot build.
