Send me all your parts for free if I'm wrong, but don't LA engines and Magnums use different pistons? I'm too tired to look right now, but I think that's the case.
As far as the crank material goes, I don't think it makes a lick of difference for what you're doing... but, look at the parting line that runs lengthwise through the crank. If it's a thin line, you've got a casting. If it's wide (and I mean, like 1/4" or so) and ground down, it's a forging. From the pic you took, it looks more like a cast unit to me. It might make a difference when you have it balanced (you know you absolutely have to do that, right?), but even the cast crank will outlive the block when it comes to ridiculous power outputs so no worries about strength.
As far as the offset water neck (which you say isn't a problem anymore anyhow), virtually any '80s 360 or 318 four-barrel motor has that offset water neck, and will kick the shit out of the Performer on the dyno, with the possible exception of using stone-stock 318 heads. The minor weight difference does not overcome the large HP deficit on even a 318; I can only imagine the appalling power loss on a 408. The '80s 4v intake is actually a very decent piece, particularly if you block off the EGR and remove the standpipe in the plenum floor (plug the hole).
I have not yet found a reason to use an LA Performer unless you've got a stock 2v 318 and happen to have one lying around, or you score it cheap and can profit by exploiting the uninformed.
