is it worth installing cam in stock 318?

Well if it is running and in good condition, your worries are over LOL! Really.... rebuildable 340 cores have asking prices by themselves of $1000 or more. So this sounds like a DEAL with the trans and bellhousing, flywheel, linkage, pedals, etc. And it sounds like you will really appreciate the 4 speed. Just be aware of the possibility that it is run out and needs a rebuild, may need a new clutch (easy to swap out), trans work etc..

A 340 in a car adds value that is hard to quantify. But for the non-engine knowledgeable, the 340 mystique is crazy. When my son mentions that he put a 340 in his '65 'Cuda, most people just start drooling, smiling, and babbling....LOL...even though it has Edlebrock heads and intake, a 273 crank, aftermarket rods and pistons, and so on. IT'S A 340 BY GAWD!!!