Having had a hopped up \6 in my 68 Barracuda for 5 years and 30k miles it is going to take a lot more work on a \6 than a cam and 2bbl to get to the same power level as a 318.
My \6 had the head shaved 0.060 to raise the compression, an Offy 4bbl intake, a 390 cfm 4bbl, 6 into 2 headers, dual exhaust and a Comp 252S cam. It was getting close to a 318 but I would have had to go bigger on the valves and do some porting to match or exceed a 318.
Also, you won't be giving up much in the way of gas mileage either. The setup I described above got better mileage than the stock engine with a 1 bbl. I got 22-23 mpg on the hiway and 19-20 around town. I have 360 now that makes close to 400 HP and it get 17-18 mpg around town and 21-22 on the hiway, this is with the same A833OD tranny and 3.55 gears.
If you have the 318 and tranny go for the swap. But before doing the swap if your car still has 9" drum brakes and a 7.25" rear axle I would upgrade those before thinking about the engine swap.