If he's havin to get heads for free, do you think he can swing almost 500 bucks for pistons? Then pay a machine shop to properly set up the necessary top ring gaps for them? No.
Look kid, we're trying to tell you the cheapest way. You're fightin us at every turn. J heads will lower compression on a 318 and the fact that they are ported will kill it even more. Save up some money and get some better heads. It's not like you gotta have this done tomorrow. Unless you fall into a pile of money and can suddenly afford some KB pistons, sell the J heads and buy what you need. The only way to get compression up on a stock pistoned 318 is to put a small chamber head on it. I don't understand why that's so difficult to get through to you. Compression = power and a stock 318 doesn't have much of either, but it can, if you'll just listen. Gettin J heads for free means you're listenin to the wrong people. If you want the simplest swap, sell the J heads and find some good, uncracked 302s. That will put you having an all LA engine and will be the easiest thing to learn on. The magnum stuff will work too, but it's more complex and requires custom pushrods which will drive the price up more. I know. I just bought some for 117 bucks. It's your motor. Don't let us tell you how to build it. You want to put J heads on it, go for it. But it's not our fault when your 318 won't pull a greasy string outta a cats ***.
If your gonna now rebuild your engine and use KB167 and zero deck your block and use around 0.025" gasket and your 72ish cc J heads will give you 9:1 CR add a high lift 268 cam and a Eddy Airgap Rpm intake and 650 Holley you'll have 350hp.