Should I chance it?

I put Magnums on a 1975 360 LA block for my '70 Duster, I hope to have it fired up the first time this weekend... I'll agree with what has been said, if they check out good with no cracks go ahead and run them as long as you're willing to pay for new pushrods and intake (or have the intake holes re-drilled for LA pattern). Also make sure your current lifters have the hole in the plunger to pump oil through the pushrods, if not you'll need a set of those too. I also agree it'll be mostly the 1.6 rockers that make the difference in power from a set of 360 heads, but I'm assuming you have stock heads on your 318 so the bigger ports and valves will make a bigger increase...

One thing to watch out for, there were two main castings of Magnum heads that had slightly different size ports and chambers. One came on earlier factory 5.9L engines, and the other came on 5.2L Magnums and was also the replacement spec'd for all engines; after a certain year they all switched to the 5.2L castings. The 5.9L ones have slightly bigger ports and chambers than the 5.2's, so for street use on a 318 I'd say the 'smaller' heads are probably better.