Swap or rebuild?

Hi guys. I picked up a 72 Swinger with a 318 and Edelbrock 4bbl. Right now it has a 7 1/4 with a 2.76 surgrip. Working on getting stuff together for this winter.
It will have a 8 3/4 with 3.23 gears. I had a 340 years back, I am looking for that kind of performance. I have about $1000 for the engine. The 318 runs good and burns no oil.
I can find 5.9 Magnums with around 120k on them for about $400. Would I be better off working my 318 or trying to drop in a 5.9? :banghead:
I don't want to have to rebuild the bottom end if I get a Magnum. Thanks.