valve springs ??

Does it have rotators on the exh. valves? If so they may be different height. I haven't worked on any Mopars with rotators but I know they used them in trucks. Generally if they don't have rotators they are the same height. Usually around 1.68" installed height I believe. Do you have a way to check the installed height?

Also if your 360 springs are much heavier and the lifters are weak in the 318 that could possibly cause them to bleed down when it's shut off. 10-15 seconds isn't running long enough to pump the lifters back up if their weak and bled down. Try running it for about a minute and see what happens. It may quiet down.