Dumb questions #178, 179, 180 I've asked this year...

I bought a 71 318 with j heads.. I was told it was a running engine, I'm assuming running good. He pulled it in favor of a 340 he found. Anyway, along with the j heads, it has an Edelbrock Performer RPM intake and an Edelbrock 600cfm... No idea what the internals are...although, I can see it has a double roller chain that feels good and tight. My question is, I have a fresh set of 302 casting heads, an Edelbrock LD4B intake with a Edelbrock 650cfm laying around..should I install my setup in place of the j heads and currently installed intake and carb..? Not knowing the cam situation, would I need to worry about my fresh set of 302's and the stockish springs..? Also, I've seen some confusing threads about what head gaskets to use.. Is there really a major concern when it comes to picking gasket thickness..? "Back in the day" this was never even a question... just went to the local parts store and asked for this part for this engine, etc..
I have a Mopar gasket kit that came with the 302's I bought, but the thickness is .050" to .053"..and I've seen where people recommend .028.. Where did simplicity of the old days go..???