273 302 318 340 build

As to your Scr, I agree with Nm9 and others,
The 318 for street and fuel economy,needs a compression boost. I would urge you to put some much taller pistons in there.At $65 per head for machining, and maybe another $65 to fit the intake to the new head height, you already have a good chunk of a new set of pistons/rings.
I gotta tell you at 120psi, that teener is not much of a puller.Putting a cam in it will only make it worse. It will need a 2800TC minimum to get moving, and wants a starter gear up near 10 or 11 to accelerate briskly. And the whole reason for this is cuz at 120 psi the teener is lazy as all get out in the torque making business, and so it has to be bandaided for street duty. So by the time you add the cost of a new TC and 3.91/4.10s, you are way beyond what you need, to get the compression up, with pistons. With compression up closer to 150/160, that teener will have a whole new personality. You will be able to do a lot more with a lot less cam. Less cam means less TC and less gear required. If the 3.23s are staying, then you need pressure if you want performance at zero to 30mph.
But keep in mind building psi is a joint venture with compression and the intake valve closing event.So you have to manipulate them both to arrive at a satisfactory conclusion.
For a streeter,a 268* might sound good at idle, but IMO is not the right cam for most guys. We hear it here all the time where guys just want to sound fast. So they slam a 268 in there and then the stock teener blows them off to nearly 30 mph. Why is that? Cuz the stocker makes 135/140ish pressure and the 268* is down at 110 or less. They skipped the pistons, and now it's back to the drawing board.
So again I urge you to reconsider boosting the compression, and selecting a conservative cam that will maximize your Dcr for the available fuel. And another thing about that is to keep in mind that you only need just enough octane to suppress detonation and no more. It would be entirely feasible to pump up the psi for 91 when using the secondaries, and then run 87 while cruising the hiways, on the transfers.
If you get this right, those extra psi will save you money on every mile it cruises. If you burn a half a tank of 91 on the weekend to satisfy 160psi,and then burn thru another half a tank of 87 going back and forth all week,figure it out. Then compare the cost of that to the the same deal on 120psi but with now running 87 full time, and getting a buncha less mpgs. Oh and a buncha less torque,power
Those pistons are getting cheaper with every mile that car burns gas.
I just can't stress this enough.