is it worth installing cam in stock 318?

Maybe instead of wasting 500 or more bucks just in cams and springs, and another 500 in header with shipping, it will make more sense to offer the previous owner another 1500 for the other '73 Demon he have with the 340 + standard tranny. At least its a real Demon then.
Depends on the engine and trans condition....is it a good engine ready to go, or a rebuildable 'core', or totally trashed out? How is the body and other parts? It could be a deal, or not. If it includes the bellhousing and clutch pedals and complete linkage, and the engine is a good core and you get a lot of other parts that you can use for spares or sell off, then it may be a good deal. But at that price, I would be expecting a full engine rebuild. I get the impression that you can't afford that.