As in stock long blocks 5.2l magnum definitely has ton advantage over LA 318 cures the two out of three main problems of the LA 318 decent CR and heads capable of 400 + hp instead of 300 + hp, third being bore size which is less of a deal especially under 400hp. So ya just a similar cam 4bbl and headers is gonna be a 100 hp difference between the two. You could always throw on a set of junkyard Magnum heads on a LA 318 and be basically in the same place.
As for 5.9 vs 360 stock the magnum has the CR advantage heads the 1.88 valved 360 has a slight disadvantage magnum 400 + hp possible 360 heads 375 ish especially with the lower cr. But if doing full build either is a good starting place.