318, 340, or 360?

If my car didn't come with a 380hp create engine which is a stock magnum long block with 288/292 cam and M1 single plain.

I would of done a magnum with probably 265/275 ish cam and airgap. Two things that would of made this engine subjectivity more streetable. Also would of port the heads to makeup for the smaller cam and probably end up with similar hp as I have but more of a driver which is what I wanted.
And with your engine even if you didn't port the heads you'd probably end up with the advertised 380 hp since that they dyno more like 410-420. And the 300 hp create with stock cam dyno 320hp so you should pickup 40 hp And with little loss to bottom end maybe even a modest gain.

As for my engine I plan to stick with the cam and add Rhoads lifter to get best of both worlds and port the heads. Also add 1.7 roller rockers which was suggested to me get even more top end. To me you should try to build the longest powerband you can but without moving it too far from idle. And if I can get that to hook up add an airgap if not stick with the single plain.

That's the beauty of these engines doesn't take a lot to get something decent even stock with manifolds they make 300hp gross. Like AJ@ said don't need much with an early A to have fun could even start with a 5.2 magnum similar potential but wouldn't recommend basically cause a 318 needs 0.5:1 more gear ratio to equal a similar built 360 and it's hard enough to get people to run more than a 3.55 gear.