ALL 318s includeing the old poly 318s all the way to the last 01 318 magnum are ALL internally balanced. The entire rotateing assembly is the same, same crank, same rods(except machined a hair skinner at the small end), same pistons... as the LA and the block is no more weak or strong, I think the only reason people put the mag heads on the older ones is a doller saveing move. If they already have a good motor and want to bolt on a few hp then the mag heads make great sense. On the other hand if you are buying a motor from a JY to throw in your ride the 5.9 is the same price and a lot of people do throw those in old cars.

The 318 mag is a great swap also it should be an easy 300 hp with out pulling the valve covers, just add a RPM air gap intake and some decent headers and there you are.

The 3100 springs are a good upgrade though as the stock 318 cam witch is a better cam the the 5.9 cam is already on the verge of loseing control of the valves around 5000 RPM. If you are planning on useing the mag heads on the 340-360 later then it is not even a waste of $$$, same thing with the intake. The cam is going to be a little different from the magnum to the bigger motors so I woudn't bother unless I was buying the cam for the bigger motor and just live with a little less tq from the 318. Might be just me but I hate buying stuff twice