No way no how is a 360 57# heavier than a 318. 5# maybe. The crank is a tad heavier, the pistons a tad lighter I have two blocks here that are both 165# on my scale bare. Nothing else is really different, both are 97 magnums.

The 318 mag pistons were .037 ish down the hole and .049 thick graphite like factory head gaskets. The 360 about the same but a big dish in the piston to make things wose. The 318 mag has a higher compression ratio AND a better cam (more lift less duration). I have driven a ton of the 5.2 and 5.9 magnum trucks new when I worked at the dealer and some 5.2s were fast and some 5.9s were weak, one motor to the next could have very different performance, you could never be sure witch one you had till you popped the hood.

None of the 5.9 trucks I have owned seem to be significantly worse MPG (just a tiny bit)than the 5.2 trucks, they were all crappy MPG except the one 318 I zero decked the block and ran .039 head gaskets, it made a huge differance, way more than I predicted or even dreamed of. I bet a 5.9 would respond the same way especially with a 5.2 cam.

Another problem is the 518 tranny, it is big heavy and sucks power out of them, one time I swapped a 904 into a dakota and the power difference was huge. An a-500 would be the real ticket behind the 5.9, Lower gears, lower weight, lower drag...

It is all about the complete package. The early 318 TBI motors had a great package, a-500 higher compression, small (tiny) cam, and the TBI does a better job with the fuel than the MPI because it has more time to va-poo-rize. Can't compare it to a 360 without looking at the whole package.