is it worth installing cam in stock 318?

Well... Here's what I'd do..

Buy the 340 and 4 speed. If it's good to go, then drop it in. If it needs a full rebuild, stick it in the corner of the shop for now. Find a camshaft you want for the 340, and stick it in the 318 with the appropriate valve springs. Get some cheap headers and put them on the 318 and run dual exhaust. Carb and intake from the 340? That'll fit the 318 too. Get a neutral balance flywheel and put the 4 speed behind the 318 since a '73 340 is externally balanced, unlike the 318.. FYI the input shaft of the 4 speed might not fit in the 318, in that case you'll need a needle bearing (national part #FC69907, should cost like $14.), instead of a pilot bushing.

If you do this you'll get the sound that you want out of the 318 for now, and you'll also have a 4 speed. When you get the time and money to rebuild the 340, it'll be there waiting for you. They're not getting any cheaper so you might want to get it while you can. The parts you bought for the 318 you can use with the 340 so your money isn't wasted.

This is pretty much what I did. Started with a 225 and 3 speed stick knowing that I wanted a 340 and 4 speed in the end. Found a cheap running 318 and did some upgrades, dropped it in place of the 225, and drove it while I looked for a suitable 340. Started with a bare 340 block, gathered all the parts I needed, then used parts from the 318 to help build the 340. Then I swapped the 340 in and put my relatively complete 318 on an engine stand. Then I found a 4 speed and swapped it in. So now I have a 340 4 speed Dart, and a 318 and spare transmission that I can use if something breaks. I couldn't afford to do everything all at once (broke college student) so I pieced it together over a few years.

Or you can just stick a big cam in the 318 and drive it as-is and enjoy the new sound. There are plenty of ways you could go about doing this. Do whatever makes you happy.