360 pros & cons vs 318

Swap out the front disks for 9" drums. THAT should scare you without adding any power. :)

The cam you have is pretty mild, and on a 318 you have more head than you need. There are certainly better 318 cams out there (XE 268, Voodoo 60403) I agree with the 5.9 Magnum idea. Junkyards are full of them, they have roller cams, and the heads are already better than you'll ever make 596's.

To swap in a big block at a minimum you'll need an A727-B transmission (getting costly as newest one is 33 years old.) Conversion mounts or rare big block K-member. New expensive headers. Big block engine. Bigger radiator. Also keep in mind those 413 heads are among the worst B/RB heads out there. But, spinning the tires (if that's really important to you) will certainly be easier after adding another 200# to the front end. If you're heart is set on a big block, start with a 440. Weigh's the same and makes more power.

No idea what your budget is, but figure $3000+ to get the 413 in and running. For the same cash you can probably freshen up a 5.9 Magnum, buy a good converter, low gear set for the 904, and run 13's.