There is no such thing as a "low cost, quick motor swap." ANY engine swap into a pre-'55 vehicle will take months and thousand(s) of dollars. If its running, leave it alone.
Yeah, you could pull the 350 out of an early '80s pontiac or chevy, but it'd likely need a rebuild. Then you've got to change motor mounts, tranny, x-member, shift linkage, driveshaft, rear axle, front springs, move steering box, fab rear brake lines, beat the tranny tunnel up so a newer tranny wil fit, etc etc etc. There are suspension and motor mount kits available, but its still expensive and time consuming. (But it is fun.)
You almost never get your money out of a V8 swap. Either you do a hack job and ruin the truck, or you put more money in the swap than the resale value of the truck. Ask 5-7 how much money he's got in his '56.
If you want a V8, buy a newer chevy truck that came with a V8 option, as the swap is easier.
