You can't get there with a 318,on as tight a budget as you seem to have, and still have a driveable car.
You can put a cam in there and make 250/280, BUT you will have that power up around 5500rpm. But at idle to 2500rpm you will have less power than you do now.So much less that when you hit second gear, you will be crying great big tears of disappointment, cuz your old slanty will have about the same power in that range.
The problem is that the stock teener has a hard time making 8/1 compression. It runs not bad with the factory cam, but any performance cam you put in there, will just drop the effective compression ratio, and that very quickly spoils the low-rpm power output.
To make a teener GO with a bigger cam means the compression has to come along; and the bigger the cam the more compression it needs, and very soon you run out of head flow.
To get to that 350hp will require a complete rebuild with new pistons to match the cam, to match the heads,plus all the other bits.And if you go to that expense, it makes no sense to do it to a 318. It will actually be cheaper to build the 360, using the same Hi-perf heads and cam, and you end up with a solid 40hp more in the 360, that's just the way it is.And the 360 will have gobs and gobs more power, right from idle.
If you're married to the teener,do yourself a favor, stop thinking about how much power it will make. It really doesn't matter with a street driven car. Just put in in there with a 4bbl and hi-flo exhaust, and go have some fun.
Why doesn't it matter? cuz from zero to 60 there is so little difference in time to get there with any sized SBM, it becomes hard to measure.If you throw some TM(TorqueMultiplication) at the little teener, it might even beat a bigger SBM, that runs less gear.
Where a 360 shines, is it doesn't need all that TM.And so it can cruise the hiway at waaay less rpm, and get better fuel-economy, and last thousands of miles longer.
Your 318 with the od trans will want 3.91s, to hit 60@5000 in second gear. Sorry about first gear running out at 32 mph, that's just the way it is with that box.