Rod and main bearings are all the same. You can use LA cam bearings in a magnum but not the other way, some don't have the holes to oil the rocker shaft. If you are putting magnum heads on an LA then you can use the magnum cam bearings also.

Timing gears and chains are completly interchangeable, even the poly 318. When I put a timing chain in either one I always tell the parts guys I am building a 340 and they give me a double roller chain for pretty cheap.

On the distributer shaft if you are talking about the intermediate shaft between the cam and oil pump, they are all the same, even the old poly 318.

Freeze plugs are all the same size in the block. Some locations are in slightly different places but the sizes are all the same. I think the heads use slightly different size plugs.

Pushrods are 3 different lengths. LAs used one length for flat tappet cams, another for the later 80s roller motors and the magnums used a 3rd length that was also hollow. If you want to put magnum heads on an LA block with a factory roller cam set up, just use the magnum PRs. If you want to put the magnum heads on a motor with flat tappet cam you need to put custom PRs 7.625 inches in it, mopar performance sells some specifically for this. In either case, roller or flat tappet you need to make sure the lifters have a hole in the center of the cup to oil the rockers through the pushrods and the pushrods need to be hollow also.